Deferred response to a prefetch request

ABSTRACT

Modifying prefetch request processing. A prefetch request is received by a local computer from a remote computer. The local computer responds to a determination that execution of the prefetch request is predicted to cause an address conflict during an execution of a transaction of the local processor by comparing a priority of the prefetch request with a priority of the transaction. Based on a result of the comparison, the local computer modifies program instructions that govern execution of the program instructions included in the prefetch request to include program instruction to perform one or both of: (i) a quiesce of the prefetch request prior to execution of the prefetch request, and (ii) a delay in execution of the prefetch request for a predetermined delay period.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of computer memorymanagement, and more specifically to techniques for improving theefficiency of transactional memory operations.

Many computer systems employ cache memory to speed data retrievaloperations. Cache memory stores copies of data found in frequently usedmain memory locations. Accessing data from cache memory speedsprocessing because cache memory can typically be accessed faster thanmain memory. If requested data is found in cache memory, then it isaccessed from cache memory. However, if requested data is not found incache memory, then the data is first copied into cache memory and thenaccessed from the cache memory.

Multi-level cache is a structure in which there are multiple cachememories. For example, a computing system may have three levels, i.e. anL1 cache, an L2 cache, and an L3 cache. Typically, in a multi-levelcache configuration, L1 is the smallest and with a short access time. Ifrequested data is not found in L1 cache, the system searches the L2cache, which is usually than L1 cache and physically further away thanthe L1 cache, thus, with a greater access time. In a similar fashion, ifthe data is not found in the L2 cache, the L3 cache is searched. Mainmemory is only accessed if the requested data is not in the L1, L2, orL3 caches. There are many different implementations of cache memory.

To improve performance, a computer architecture often includes prefetchinstructions that can move data from main memory or a lower cache levelto a higher cache level (closer to the processor) in anticipation of anaccess to the data. The execution of a prefetch instruction can bespeculative in nature in that it may be performed before it is knownthat the data moved by the prefetch instruction will actually beaccessed. Data in anticipation of either a read access or a write accessmay be prefetched, which is called a read prefetch or a write prefetchrespectively. Cache coherency protocols in computers, that enable thesharing, synchronization, and parallel processing of data, often requirea processor to gain ownership of the data before it can be written to(i.e., changed). Because the data may be currently owned by anotherprocessor, gaining ownership can be a lengthy process. The concept ofownership is necessary to prevent data that is being read by oneprocessor from a memory location from being changed by another processorthat writes to the same location or a different location (if copies ofthe data are in multiple different locations). Therefore it is oftenadvantageous for a processor to execute a write prefetch instruction togain ownership of the data in anticipation of writing to the data sothat processing is not delayed while ownership is acquired.

Since the access time of a cache is often critical to the performance ofa code that is executing, and a cache is often busy with many operations(e.g., servicing misses), it is beneficial to decrease a cache'sworkload, if possible. One common technique used to decrease a cache'sworkload includes accumulating multiple stores that store into to acommon cache line in a cache line buffer, and then storing the contentsof the cache line buffer into a cache as a single operation. Thisdecreases a cache's workload and improves its response time and, thus,potentially improves the performance of a code that is executing. Such atechnique is commonly performed in a mechanism called a store cache.

Transactional memory is a type of memory operation that groups one ormore load and store operations performed by a processor into a singletransaction that is visible to other processors as a single operationwhen the transaction completes. The effects (e.g., the data) of multiplestore operations participating in the single transaction are not madevisible to other processors until the transaction is complete. Atransactional load is a load that accesses and buffers data until atransaction completes, after which the data is forwarded to theprocessor that requested it. If the data from a transactional load ischanged (i.e., its location in memory written to) after the load occursand before the transaction completes, the transaction is aborted. Atransactional write is a write whose data is not seen until thetransaction completes. A read from or a write to a location that is thetarget of a transactional write in a transaction aborts the transaction.Transactional memory is often helpful in synchronizing work that isperformed in parallel on multiple CPUs (by enabling atomic operations onan arbitrary set of memory locations) and when multiple writes must notbe interrupted. Since writes that participate in a transaction are notvisible until the transaction completes, a read from a location that isgoing to be written by a write in the transaction aborts thetransaction.

SUMMARY

Embodiments of the present invention provide a method, computer programproduct and a computer system to modify prefetch request processing.

In a first embodiment, a method includes: receiving, by a localprocessor of a group of one or more processors of a local computer, aprefetch request from a remote processor of a remote computer that is incommunication with the local computer; prior to execution of programinstructions included in the prefetch request, determining, by the groupof one or more processors, whether execution of the prefetch request ispredicted to cause an address conflict by accessing a memory addressthat is accessed during an execution of a transaction of the localprocessor; responsive to a determination that execution of the prefetchrequest is predicted to cause a conflict during the execution of thetransaction, comparing, by the group of one or more processors, apriority of the prefetch request with a priority of the transaction; andbased, at least in part, on a result of the comparison, modifying, bythe group of one or more processors, one or more program instructionsthat govern execution of the program instructions included in theprefetch request to include program instruction to perform one or bothof: (i) a quiesce of the prefetch request prior to execution of theprefetch request, (ii) a delay in execution of the prefetch request fora predetermined delay period.

In a second embodiment, a computer program product includes one or morecomputer readable storage media with program instructions to modifyprefetch request processing stored on the one or more computer readablestorage media. The program instructions comprising: program instructionsto receiving, by a local processor of a group of one or more processorsof a local computer, a prefetch request from a remote processor of aremote computer that is in communication with the local computer;program instructions to, prior to execution of program instructionsincluded in the prefetch request, determine whether execution of theprefetch request is predicted to cause an address conflict by accessinga memory address that is accessed during an execution of a transactionof the local processor; program instructions to respond to adetermination that execution of the prefetch request is predicted tocause a conflict during the execution of the transaction, comparing apriority of the prefetch request with a priority of the transaction; andprogram instructions to, based, at least in part, on a result of thecomparison, modify one or more program instructions that governexecution of the program instructions included in the prefetch requestto include program instruction to perform one or both of: (i) a quiesceof the prefetch request prior to execution of the prefetch request, and(ii) a delay in execution of the prefetch request for a predetermineddelay period.

In a third embodiment, computer system to modify prefetch requestprocessing includes one or more computer processors, one or morecomputer readable storage medium, and program instructions stored on thecomputer readable storage medium for execution by at least one of theone or more processors. The program instructions comprising: programinstructions to receiving, by a local processor of a group of one ormore processors of a local computer, a prefetch request from a remoteprocessor of a remote computer that is in communication with the localcomputer; program instructions to, prior to execution of programinstructions included in the prefetch request, determine whetherexecution of the prefetch request is predicted to cause an addressconflict by accessing a memory address that is accessed during anexecution of a transaction of the local processor; program instructionsto respond to a determination that execution of the prefetch request ispredicted to cause a conflict during the execution of the transaction,comparing a priority of the prefetch request with a priority of thetransaction; and program instructions to, based, at least in part, on aresult of the comparison, modify one or more program instructions thatgovern execution of the program instructions included in the prefetchrequest to include program instruction to perform one or both of: (i) aquiesce of the prefetch request prior to execution of the prefetchrequest, and (ii) a delay in execution of the prefetch request for apredetermined delay period.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts an example multicore transactional memory environment, inaccordance with an illustrative embodiment;

FIG. 2 depicts an example multicore transactional memory environment, inaccordance with an illustrative embodiment;

FIG. 3 depicts example components of an example CPU, in accordance withan illustrative embodiment;

FIG. 4 depicts a block diagram of a portion of a computing system, inaccordance with an embodiment of the present invention;

FIG. 5 depicts a cache system depicted in FIG. 4, in accordance with anembodiment of the present invention;

FIG. 6 depicts a format of a read request and a response to a readrequest, in accordance with an embodiment of the present invention;

FIG. 7 depicts a format of a prefetch request, in accordance with anembodiment of the present invention;

FIG. 8 depicts a format of a packed prefetch request, in accordance withan embodiment of the present invention;

FIG. 9 depicts a flow chart for an operation of transactional memorysystem depicted in FIG. 5, in accordance with an embodiment of thepresent invention;

FIG. 10 depicts a flow chart for an operation of prefetch managerdepicted in FIG. 5, in accordance with an embodiment of the presentinvention; and

FIG. 11 depicts a block diagram of a computer system that incorporatesthe cache system that is depicted FIGS. 4 and 5, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

Detailed embodiments of the present invention are disclosed herein withreference to the accompanying drawings. It is to be understood that thedisclosed embodiments are merely illustrative of potential embodimentsof the present invention and may take various forms. In addition, eachof the examples given in connection with the various embodiments isintended to be illustrative, and not restrictive. Further, the figuresare not necessarily to scale, some features may be exaggerated to showdetails of particular components. Therefore, specific structural andfunctional details disclosed herein are not to be interpreted aslimiting, but merely as a representative basis for teaching one skilledin the art to variously employ the present invention.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

A memory hierarchy in a modern computer often includes multiple layersof cache, some layers dedicated to, and accessible by, a singleprocessor and other, lower and larger layers of cache accessible bymultiple processors. A cache often provides a quick access to recentlyaccessed data or to data near recently accessed data. Caches are givenlabels in their order of logical position relative to a given processor,L1, L2, L3, etc., with the L1 cache logically nearest to the processor.This ordering is also the order in which the caches are accessed whenthe processor is attempting to read data. L1 is first accessed for thedata, then L2 if the data is not found in L1, and so on. An L0 cache issometimes employed that is small and tightly integrated with theprocessor, often providing 1-cycle access. An L0 cache, if it exists, islogically closer to the processor than an L1 cache. The levels of cachein a computer system, together with the main memory (often a largedynamic RAM), constitute the memory hierarchy of the computer system. Inthe context of a memory hierarchy, the term “below” means logicallyfurther away from the processor.

Many techniques have been developed to increase the efficiency of amemory hierarchy. Memory hierarchy efficiency relative to a benchmarkprogram is usually measured by its average access time during theexecution of the benchmark program. A store cache is a technique thatimproves average access time by decreasing the workload of a cache inthe next cache layer below that of the store cache. It does this byaccumulating stores to a cache line in a buffer and then writing thecontents of the buffer to the cache line in the next layer of cache inone access, eliminating the multiple accesses that would have transpiredhad each separate store performed a store operation.

In modern computing systems that have multiple processors, there is aneffort to increase performance by computing parts of a program inparallel on the same processor, if the processor is multithreaded and/oron multiple different processors, and combine or compare results asneeded intermittently during an execution. This is often accomplished bysynchronizing multiple threads of execution on the same or differentprocessors, and/or making results produced by one thread of executionvisible to other threads of execution. Synchronization is oftenaccomplished by executing “atomic” instructions and groups ofinstructions. An atomic instruction appears to execute “all at once” toother threads of execution and to other processors, i.e., the atomicinstruction can never be observed to be partially complete. In likemanner, a group of instructions may be made atomic by making theireffect visible all at once to other threads and processors. Memoryoperations are often available in an atomic version because multiplethreads of execution often communicate and synchronize with each otherthrough values written to and read from memory locations known by allparticipating threads of execution. For example, an atomic instructionmay read a memory location, and if the memory location contains aspecific value, write another value back to the same memory location ina single atomic operation. This action would tell other processes thatmay be testing the variable (by reading it).

A modern technique that may improve the performance of an applicationexecuting on a processor is to execute some instructions speculativelywhen there resources available to do so. Speculative execution is a termthat refers to an execution that is probably going to happen in thefuture, but may not actually happen. This usually occurs when a branchinstruction is seen in the instruction stream and its behavior ispredicted based on previous behavior, because the information thatdetermines its actual behavior is not yet available. Instead of waitingfor this information to become available, the processor can do workbased on a predicted path that this branch will probably take, andexecute instructions on this predicted path. The instructions executedon this predicted path are speculative instructions—until it is knownthat they are on an actual path of execution, i.e., that the branch waspredicted correctly. If it turns out that these speculative instructionsare on the wrong actual path of execution (wrong prediction), theeffects of these instructions must be squashed, i.e., eliminated,undone, and not be visible to other processors.

Instructions that were executed speculatively but should not have beenexecuted can be a demanding design challenge, and is especiallydemanding regarding speculative store instructions that should not havebeen executed. If a store that should not have been executed stores datato memory, it may overwrite data that should not have been overwritten,and the written data may be read and used by the same or a differentprocessor in an application. Computer architectures solve this problemby not executing speculative store instructions, or by executing themspeculatively and storing store results in a store buffer or store queueuntil the in-order point of the instruction (which decreasesperformance). An alternative approach is to prevent other threads ofexecution from seeing the data produced by a speculative store until itis known to be on the correct path, and to delete the data if it is aresult of a store on an incorrect path.

Simultaneous multithreading is a technique often incorporated in modernprocessors that enables a single processor to execute multipleapplications concurrently (or multiple parts of the same application),with each application having its own thread of execution. The singleprocessor fetches instructions on each thread separately and executesthe instructions on shared execution units (e.g., adders, multipliers,etc.) within the processor, all the while keeping track of whichinstructions belong to which thread. If such treads are synchronizingtheir work with other threads, they often do so via the execution ofatomic instructions, or groups of instructions, whose execution is madeatomic.

Historically, a computer system or processor had only a single processor(aka processing unit or central processing unit). The processor includedan instruction processing unit (IPU), a branch unit, a memory controlunit and the like. Such processors were capable of executing a singlethread of a program at a time. Operating systems were developed thatcould time-share a processor by dispatching a program to be executed onthe processor for a period of time, and then dispatching another programto be executed on the processor for another period of time. Astechnology evolved, memory subsystem caches were often added to theprocessor as well as complex dynamic address translation includingtranslation lookaside buffers (TLBs). The IPU itself was often referredto as a processor. As technology continued to evolve, an entireprocessor could be packaged on a single semiconductor chip or die, sucha processor was referred to as a microprocessor. Then processors weredeveloped that incorporated multiple IPUs, such processors were oftenreferred to as multi-processors. Each such processor of amulti-processor computer system (processor) may include individual orshared caches, memory interfaces, system bus, address translationmechanism and the like. Virtual machine and instruction set architecture(ISA) emulators added a layer of software to a processor, that providedthe virtual machine with multiple “virtual processors” (aka processors)by time-slice usage of a single IPU in a single hardware processor. Astechnology further evolved, multi-threaded processors were developed,enabling a single hardware processor having a single multi-thread IPU toprovide a capability of simultaneously executing threads of differentprograms, thus each thread of a multi-threaded processor appeared to theoperating system as a processor. As technology further evolved, it waspossible to put multiple processors (each having an IPU) on a singlesemiconductor chip or die. These processors were referred to processorcores or just cores. Thus the terms such as processor, centralprocessing unit, processing unit, microprocessor, core, processor core,processor thread, and thread, for example, are often usedinterchangeably. Aspects of embodiments herein may be practiced by anyor all processors including those shown supra, without departing fromthe teachings herein. Wherein the term “thread” or “processor thread” isused herein, it is expected that particular advantage of the embodimentmay be had in a processor thread implementation.

Transaction Execution in Intel® Based Embodiments

In “Intel® Architecture Instruction Set Extensions ProgrammingReference” 319433-012A, February 2012, incorporated herein by referencein its entirety, Chapter 8 teaches, in part, that multithreadedapplications may take advantage of increasing numbers of CPU cores toachieve higher performance. However, the writing of multi-threadedapplications requires programmers to understand and take into accountdata sharing among the multiple threads. Access to shared data typicallyrequires synchronization mechanisms. These synchronization mechanismsare used to ensure that multiple threads update shared data byserializing operations that are applied to the shared data, oftenthrough the use of a critical section that is protected by a lock. Sinceserialization limits concurrency, programmers try to limit the overheaddue to synchronization.

Intel® Transactional Synchronization Extensions (Intel® TSX) allow aprocessor to dynamically determine whether threads need to be serializedthrough lock-protected critical sections, and to perform thatserialization only when required. This allows the processor to exposeand exploit concurrency that is hidden in an application because ofdynamically unnecessary synchronization.

With Intel TSX, programmer-specified code regions (also referred to as“transactional regions” or just “transactions”) are executedtransactionally. If the transactional execution completes successfully,then all memory operations performed within the transactional regionwill appear to have occurred instantaneously when viewed from otherprocessors. A processor makes the memory operations of the executedtransaction, performed within the transactional region, visible to otherprocessors only when a successful commit occurs, i.e., when thetransaction successfully completes execution. This process is oftenreferred to as an atomic commit.

Intel TSX provides two software interfaces to specify regions of codefor transactional execution. Hardware Lock Elision (HLE) is a legacycompatible instruction set extension (comprising the XACQUIRE andXRELEASE prefixes) to specify transactional regions. RestrictedTransactional Memory (RTM) is a new instruction set interface(comprising the XBEGIN, XEND, and XABORT instructions) for programmersto define transactional regions in a more flexible manner than thatpossible with HLE. HLE is for programmers who prefer the backwardcompatibility of the conventional mutual exclusion programming model andwould like to run HLE-enabled software on legacy hardware but would alsolike to take advantage of the new lock elision capabilities on hardwarewith HLE support. RTM is for programmers who prefer a flexible interfaceto the transactional execution hardware. In addition, Intel TSX alsoprovides an XTEST instruction. This instruction allows software to querywhether the logical processor is transactionally executing in atransactional region identified by either HLE or RTM.

Since a successful transactional execution ensures an atomic commit, theprocessor executes the code region optimistically without explicitsynchronization. If synchronization was unnecessary for that specificexecution, execution can commit without any cross-thread serialization.If the processor cannot commit atomically, then the optimistic executionfails. When this happens, the processor will roll back the execution, aprocess referred to as a transactional abort. On a transactional abort,the processor will discard all updates performed in the memory regionused by the transaction, restore architectural state to appear as if theoptimistic execution never occurred, and resume executionnon-transactionally.

A processor can perform a transactional abort for numerous reasons. Aprimary reason to abort a transaction is due to conflicting memoryaccesses between the transactionally executing logical processor andanother logical processor. Such conflicting memory accesses may preventa successful transactional execution. Memory addresses read from withina transactional region constitute the read-set of the transactionalregion and addresses written to within the transactional regionconstitute the write-set of the transactional region. Intel TSXmaintains the read- and write-sets at the granularity of a cache line. Aconflicting memory access occurs if another logical processor eitherreads a location that is part of the transactional region's write-set orwrites a location that is a part of either the read- or write-set of thetransactional region. A conflicting access typically means thatserialization is required for this code region. Since Intel TSX detectsdata conflicts at the granularity of a cache line, unrelated datalocations placed in the same cache line will be detected as conflictsthat result in transactional aborts. Transactional aborts may also occurdue to limited transactional resources. For example, the amount of dataaccessed in the region may exceed an implementation-specific capacity.Additionally, some instructions and system events may causetransactional aborts. Frequent transactional aborts result in wastedcycles and increased inefficiency.

Hardware Lock Elision

Hardware Lock Elision (HLE) provides a legacy compatible instruction setinterface for programmers to use transactional execution. HLE providestwo new instruction prefix hints: XACQUIRE and XRELEASE.

With HLE, a programmer adds the XACQUIRE prefix to the front of theinstruction that is used to acquire the lock that is protecting thecritical section. The processor treats the prefix as a hint to elide thewrite associated with the lock acquire operation. Even though the lockacquire has an associated write operation to the lock, the processordoes not add the address of the lock to the transactional region'swrite-set nor does it issue any write requests to the lock. Instead, theaddress of the lock is added to the read-set. The logical processorenters transactional execution. If the lock was available before theXACQUIRE prefixed instruction, then all other processors will continueto see the lock as available afterwards. Since the transactionallyexecuting logical processor neither added the address of the lock to itswrite-set nor performed externally visible write operations to the lock,other logical processors can read the lock without causing a dataconflict. This allows other logical processors to also enter andconcurrently execute the critical section protected by the lock. Theprocessor automatically detects any data conflicts that occur during thetransactional execution and will perform a transactional abort ifnecessary.

Even though the eliding processor did not perform any external writeoperations to the lock, the hardware ensures program order of operationson the lock. If the eliding processor itself reads the value of the lockin the critical section, it will appear as if the processor had acquiredthe lock, i.e. the read will return the non-elided value. This behaviorallows an HLE execution to be functionally equivalent to an executionwithout the HLE prefixes.

An XRELEASE prefix can be added in front of an instruction that is usedto release the lock protecting a critical section. Releasing the lockinvolves a write to the lock. If the instruction is to restore the valueof the lock to the value the lock had prior to the XACQUIRE prefixedlock acquire operation on the same lock, then the processor elides theexternal write request associated with the release of the lock and doesnot add the address of the lock to the write-set. The processor thenattempts to commit the transactional execution.

With HLE, if multiple threads execute critical sections protected by thesame lock but they do not perform any conflicting operations on eachother's data, then the threads can execute concurrently and withoutserialization. Even though the software uses lock acquisition operationson a common lock, the hardware recognizes this, elides the lock, andexecutes the critical sections on the two threads without requiring anycommunication through the lock—if such communication was dynamicallyunnecessary.

If the processor is unable to execute the region transactionally, thenthe processor will execute the region non-transactionally and withoutelision. HLE enabled software has the same forward progress guaranteesas the underlying non-HLE lock-based execution. For successful HLEexecution, the lock and the critical section code must follow certainguidelines. These guidelines only affect performance; and failure tofollow these guidelines will not result in a functional failure.Hardware without HLE support will ignore the XACQUIRE and XRELEASEprefix hints and will not perform any elision since these prefixescorrespond to the REPNE/REPE IA-32 prefixes which are ignored on theinstructions where XACQUIRE and XRELEASE are valid.

Importantly, HLE is compatible with the existing lock-based programmingmodel. Improper use of hints will not cause functional bugs though itmay expose latent bugs already in the code.

Restricted Transactional Memory (RTM) provides a flexible softwareinterface for transactional execution. RTM provides three newinstructions—XBEGIN, XEND, and XABORT—for programmers to start, commit,and abort a transactional execution.

The programmer uses the XBEGIN instruction to specify the start of atransactional code region and the XEND instruction to specify the end ofthe transactional code region. If the RTM region could not besuccessfully executed transactionally, then the XBEGIN instruction takesan operand that provides a relative offset to the fallback instructionaddress.

A processor may abort RTM transactional execution for many reasons. Inmany instances, the hardware automatically detects transactional abortconditions and restarts execution from the fallback instruction addresswith the architectural state corresponding to that present at the startof the XBEGIN instruction and the EAX register updated to describe theabort status.

The XABORT instruction allows programmers to abort the execution of anRTM region explicitly. The XABORT instruction takes an 8-bit immediateargument that is loaded into the EAX register and will thus be availableto software following an RTM abort. RTM instructions do not have anydata memory location associated with them. While the hardware providesno guarantees as to whether an RTM region will ever successfully committransactionally, most transactions that follow the recommendedguidelines are expected to successfully commit transactionally. However,programmers must always provide an alternative code sequence in thefallback path to guarantee forward progress. This may be as simple asacquiring a lock and executing the specified code regionnon-transactionally. Further, a transaction that always aborts on agiven implementation may complete transactionally on a futureimplementation. Therefore, programmers must ensure the code paths forthe transactional region and the alternative code sequence arefunctionally tested.

Detection of HLE Support

A processor supports HLE execution if CPUID.07H.EBX.HLE [bit 4]=1.However, an application can use the HLE prefixes (XACQUIRE and XRELEASE)without checking whether the processor supports HLE. Processors withoutHLE support ignore these prefixes and will execute the code withoutentering transactional execution.

Detection of RTM Support

A processor supports RTM execution if CPUID.07H.EBX.RTM [bit 11]=1. Anapplication must check if the processor supports RTM before it uses theRTM instructions (XBEGIN, XEND, XABORT). These instructions willgenerate a #UD exception when used on a processor that does not supportRTM.

Detection of XTEST Instruction

A processor supports the XTEST instruction if it supports either HLE orRTM. An application must check either of these feature flags beforeusing the XTEST instruction. This instruction will generate a #UDexception when used on a processor that does not support either HLE orRTM.

Querying Transactional Execution Status

The XTEST instruction can be used to determine the transactional statusof a transactional region specified by HLE or RTM. Note, while the HLEprefixes are ignored on processors that do not support HLE, the XTESTinstruction will generate a #UD exception when used on processors thatdo not support either HLE or RTM.

Requirements for HLE Locks

For HLE execution to successfully commit transactionally, the lock mustsatisfy certain properties and access to the lock must follow certainguidelines.

An XRELEASE prefixed instruction must restore the value of the elidedlock to the value it had before the lock acquisition. This allowshardware to safely elide locks by not adding them to the write-set. Thedata size and data address of the lock release (XRELEASE prefixed)instruction must match that of the lock acquire (XACQUIRE prefixed) andthe lock must not cross a cache line boundary.

Software should not write to the elided lock inside a transactional HLEregion with any instruction other than an XRELEASE prefixed instruction,otherwise such a write may cause a transactional abort. In addition,recursive locks (where a thread acquires the same lock multiple timeswithout first releasing the lock) may also cause a transactional abort.Note that software can observe the result of the elided lock acquireinside the critical section. Such a read operation will return the valueof the write to the lock.

The processor automatically detects violations to these guidelines, andsafely transitions to a non-transactional execution without elision.Since Intel TSX detects conflicts at the granularity of a cache line,writes to data collocated on the same cache line as the elided lock maybe detected as data conflicts by other logical processors eliding thesame lock.

Transactional Nesting

Both HLE and RTM support nested transactional regions. However, atransactional abort restores state to the operation that startedtransactional execution: either the outermost XACQUIRE prefixed HLEeligible instruction or the outermost XBEGIN instruction. The processortreats all nested transactions as one transaction.

HLE Nesting and Elision

Programmers can nest HLE regions up to an implementation specific depthof MAX_HLE_NEST_COUNT. Each logical processor tracks the nesting countinternally but this count is not available to software. An XACQUIREprefixed HLE-eligible instruction increments the nesting count, and anXRELEASE prefixed HLE-eligible instruction decrements it. The logicalprocessor enters transactional execution when the nesting count goesfrom zero to one. The logical processor attempts to commit only when thenesting count becomes zero. A transactional abort may occur if thenesting count exceeds MAX_HLE_NEST_COUNT.

In addition to supporting nested HLE regions, the processor can alsoelide multiple nested locks. The processor tracks a lock for elisionbeginning with the XACQUIRE prefixed HLE eligible instruction for thatlock and ending with the XRELEASE prefixed HLE eligible instruction forthat same lock. The processor can, at any one time, track up to aMAX_HLE_ELIDED_LOCKS number of locks. For example, if the implementationsupports a MAX_HLE_ELIDED_LOCKS value of two and if the programmer neststhree HLE identified critical sections (by performing XACQUIRE prefixedHLE eligible instructions on three distinct locks without performing anintervening XRELEASE prefixed HLE eligible instruction on any one of thelocks), then the first two locks will be elided, but the third won't beelided (but will be added to the transaction's writeset). However, theexecution will still continue transactionally. Once an XRELEASE for oneof the two elided locks is encountered, a subsequent lock acquiredthrough the XACQUIRE prefixed HLE eligible instruction will be elided.

The processor attempts to commit the HLE execution when all elidedXACQUIRE and XRELEASE pairs have been matched, the nesting count goes tozero, and the locks have satisfied requirements. If execution cannotcommit atomically, then execution transitions to a non-transactionalexecution without elision as if the first instruction did not have anXACQUIRE prefix.

RTM Nesting

Programmers can nest RTM regions up to an implementation specificMAX_RTM_NEST_COUNT. The logical processor tracks the nesting countinternally but this count is not available to software. An XBEGINinstruction increments the nesting count, and an XEND instructiondecrements the nesting count. The logical processor attempts to commitonly if the nesting count becomes zero. A transactional abort occurs ifthe nesting count exceeds MAX_RTM_NEST_COUNT.

Nesting HLE and RTM

HLE and RTM provide two alternative software interfaces to a commontransactional execution capability. Transactional processing behavior isimplementation specific when HLE and RTM are nested together, e.g., HLEis inside RTM or RTM is inside HLE. However, in all cases, theimplementation will maintain HLE and RTM semantics. An implementationmay choose to ignore HLE hints when used inside RTM regions, and maycause a transactional abort when RTM instructions are used inside HLEregions. In the latter case, the transition from transactional tonon-transactional execution occurs seamlessly since the processor willre-execute the HLE region without actually doing elision, and thenexecute the RTM instructions.

Abort Status Definition

RTM uses the EAX register to communicate abort status to software.Following an RTM abort the EAX register has the following definition.

TABLE 1 RTM Abort Status Definition EAX Register Bit Position Meaning 0Set if abort caused by XABORT instruction 1 If set, the transaction maysucceed on retry, this bit is always clear if bit 0 is set 2 Set ifanother logical processor conflicted with a memory address that was partof the transaction that aborted 3 Set if an internal buffer overflowed 4Set if a debug breakpoint was hit 5 Set if an abort occurred duringexecution of a nested transaction 23:6 Reserved 31-24 XABORT argument(only valid if bit 0 set, otherwise reserved)

The EAX abort status for RTM only provides causes for aborts. It doesnot by itself encode whether an abort or commit occurred for the RTMregion. The value of EAX can be 0 following an RTM abort. For example, aCPUID instruction when used inside an RTM region causes a transactionalabort and may not satisfy the requirements for setting any of the EAXbits. This may result in an EAX value of 0.

RTM Memory Ordering

A successful RTM commit causes all memory operations in the RTM regionto appear to execute atomically. A successfully committed RTM regionconsisting of an XBEGIN followed by an XEND, even with no memoryoperations in the RTM region, has the same ordering semantics as a LOCKprefixed instruction.

The XBEGIN instruction does not have fencing semantics. However, if anRTM execution aborts, then all memory updates from within the RTM regionare discarded and are not made visible to any other logical processor.

RTM-Enabled Debugger Support

By default, any debug exception inside an RTM region will cause atransactional abort and will redirect control flow to the fallbackinstruction address with architectural state recovered and bit 4 in EAXset. However, to allow software debuggers to intercept execution ondebug exceptions, the RTM architecture provides additional capability.

If bit 11 of DR7 and bit 15 of the IA32_DEBUGCTL_MSR are both 1, any RTMabort due to a debug exception (#DB) or breakpoint exception (#BP)causes execution to roll back and restart from the XBEGIN instructioninstead of the fallback address. In this scenario, the EAX register willalso be restored back to the point of the XBEGIN instruction.

Programming Considerations

Typical programmer-identified regions are expected to transactionallyexecute and commit successfully. However, Intel TSX does not provide anysuch guarantee. A transactional execution may abort for many reasons. Totake full advantage of the transactional capabilities, programmersshould follow certain guidelines to increase the probability of theirtransactional execution committing successfully.

This section discusses various events that may cause transactionalaborts. The architecture ensures that updates performed within atransaction that subsequently aborts execution will never becomevisible. Only committed transactional executions initiate an update tothe architectural state. Transactional aborts never cause functionalfailures and only affect performance.

Instruction Based Considerations

Programmers can use any instruction safely inside a transaction (HLE orRTM) and can use transactions at any privilege level. However, someinstructions will always abort the transactional execution and causeexecution to seamlessly and safely transition to a non-transactionalpath.

Intel TSX allows for most common instructions to be used insidetransactions without causing aborts. The following operations inside atransaction do not typically cause an abort:

-   -   Operations on the instruction pointer register, general purpose        registers (GPRs) and the status flags (CF, OF, SF, PF, AF, and        ZF); and    -   Operations on XMM and YMM registers and the MXCSR register.

However, programmers must be careful when intermixing SSE and AVXoperations inside a transactional region. Intermixing SSE instructionsaccessing XMM registers and AVX instructions accessing YMM registers maycause transactions to abort. Programmers may use REP/REPNE prefixedstring operations inside transactions. However, long strings may causeaborts. Further, the use of CLD and STD instructions may cause aborts ifthey change the value of the DF flag. However, if DF is 1, the STDinstruction will not cause an abort. Similarly, if DF is 0, then the CLDinstruction will not cause an abort.

Instructions not enumerated here as causing abort when used inside atransaction will typically not cause a transaction to abort (examplesinclude but are not limited to MFENCE, LFENCE, SFENCE, RDTSC, RDTSCP,etc.).

The following instructions will abort transactional execution on anyimplementation:

-   -   XABORT    -   CPUID    -   PAUSE

In addition, in some implementations, the following instructions mayalways cause transactional aborts. These instructions are not expectedto be commonly used inside typical transactional regions. However,programmers must not rely on these instructions to force a transactionalabort, since whether they cause transactional aborts is implementationdependent.

-   -   Operations on X87 and MMX architecture state. This includes all        MMX and X87 instructions, including the FXRSTOR and FXSAVE        instructions.    -   Update to non-status portion of EFLAGS: CLI, STI, POPFD, POPFQ,        CLTS.    -   Instructions that update segment registers, debug registers        and/or control registers: MOV to DS/ES/FS/GS/SS, POP        DS/ES/FS/GS/SS, LDS, LES, LFS, LGS, LSS, SWAPGS, WRFSBASE,        WRGSBASE, LGDT, SGDT, LIDT, SIDT, LLDT, SLDT, LTR, STR, Far        CALL, Far JMP, Far RET, IRET, MOV to DRx, MOV to        CR0/CR2/CR3/CR4/CR8 and LMSW.    -   Ring transitions: SYSENTER, SYSCALL, SYSEXIT, and SYSRET.    -   TLB and Cacheability control: CLFLUSH, INVD, WBINVD, INVLPG,        INVPCID, and memory instructions with a non-temporal hint        (MOVNTDQA, MOVNTDQ, MOVNTI, MOVNTPD, MOVNTPS, and MOVNTQ).    -   Processor state save: XSAVE, XSAVEOPT, and XRSTOR.    -   Interrupts: INTn, INTO.    -   IO: IN, INS, REP INS, OUT, OUTS, REP OUTS and their variants.    -   VMX: VMPTRLD, VMPTRST, VMCLEAR, VMREAD, VMWRITE, VMCALL,        VMLAUNCH, VMRESUME, VMXOFF, VMXON, INVEPT, and INVVPID.    -   SMX: GETSEC.    -   UD2, RSM, RDMSR, WRMSR, HLT, MONITOR, MWAIT, XSETBV, VZEROUPPER,        MASKMOVQ, and V/MASKMOVDQU.        Runtime Considerations

In addition to the instruction-based considerations, runtime events maycause transactional execution to abort. These may be due to data accesspatterns or micro-architectural implementation features. The followinglist is not a comprehensive discussion of all abort causes.

Any fault or trap in a transaction that must be exposed to software willbe suppressed. Transactional execution will abort and execution willtransition to a non-transactional execution, as if the fault or trap hadnever occurred. If an exception is not masked, then that un-maskedexception will result in a transactional abort and the state will appearas if the exception had never occurred.

Synchronous exception events (#DE, #OF, #NP, #SS, #GP, #BR, #UD, #AC,#XF, #PF, #NM, #TS, #MF, #DB, #BP/INT3) that occur during transactionalexecution may cause an execution not to commit transactionally, andrequire a non-transactional execution. These events are suppressed as ifthey had never occurred. With HLE, since the non-transactional code pathis identical to the transactional code path, these events will typicallyre-appear when the instruction that caused the exception is re-executednon-transactionally, causing the associated synchronous events to bedelivered appropriately in the non-transactional execution. Asynchronousevents (NMI, SMI, INTR, IPI, PMI, etc.) occurring during transactionalexecution may cause the transactional execution to abort and transitionto a non-transactional execution. The asynchronous events will be pendedand handled after the transactional abort is processed.

Transactions only support write-back cacheable memory type operations. Atransaction may always abort if the transaction includes operations onany other memory type. This includes instruction fetches to UC memorytype.

Memory accesses within a transactional region may require the processorto set the Accessed and Dirty flags of the referenced page table entry.The behavior of how the processor handles this is implementationspecific. Some implementations may allow the updates to these flags tobecome externally visible even if the transactional region subsequentlyaborts. Some Intel TSX implementations may choose to abort thetransactional execution if these flags need to be updated. Further, aprocessor's page-table walk may generate accesses to its owntransactionally written but uncommitted state. Some Intel TSXimplementations may choose to abort the execution of a transactionalregion in such situations. Regardless, the architecture ensures that, ifthe transactional region aborts, then the transactionally written statewill not be made architecturally visible through the behavior ofstructures such as TLBs.

Executing self-modifying code transactionally may also causetransactional aborts. Programmers must continue to follow the Intelrecommended guidelines for writing self-modifying and cross-modifyingcode even when employing HLE and RTM. While an implementation of RTM andHLE will typically provide sufficient resources for executing commontransactional regions, implementation constraints and excessive sizesfor transactional regions may cause a transactional execution to abortand transition to a non-transactional execution. The architectureprovides no guarantee of the amount of resources available to dotransactional execution and does not guarantee that a transactionalexecution will ever succeed.

Conflicting requests to a cache line accessed within a transactionalregion may prevent the transaction from executing successfully. Forexample, if logical processor P0 reads line A in a transactional regionand another logical processor P1 writes line A (either inside or outsidea transactional region) then logical processor P0 may abort if logicalprocessor P1's write interferes with processor P0's ability to executetransactionally.

Similarly, if P0 writes line A in a transactional region and P1 reads orwrites line A (either inside or outside a transactional region), then P0may abort if P1's access to line A interferes with P0's ability toexecute transactionally. In addition, other coherence traffic may attimes appear as conflicting requests and may cause aborts. While thesefalse conflicts may happen, they are expected to be uncommon. Theconflict resolution policy to determine whether P0 or P1 aborts in theabove scenarios is implementation specific.

Generic Transaction Execution Embodiments:

According to “ARCHITECTURES FOR TRANSACTIONAL MEMORY”, a dissertationsubmitted to the Department of Computer Science and the Committee onGraduate Studies of Stanford University in partial fulfillment of therequirements for the Degree of Doctor of Philosophy, by Austen McDonald,June 2009, incorporated by reference herein in its entirety,fundamentally, there are three mechanisms needed to implement an atomicand isolated transactional region: versioning, conflict detection, andcontention management.

To make a transactional code region appear atomic, all the modificationsperformed by that transactional code region must be stored and keptisolated from other transactions until commit time. The system does thisby implementing a versioning policy. Two versioning paradigms exist:eager and lazy. An eager versioning system stores newly generatedtransactional values in place and stores previous memory values on theside, in what is called an undo-log. A lazy versioning system stores newvalues temporarily in what is called a write buffer, copying them tomemory only on commit. In either system, the cache is used to optimizestorage of new versions.

To ensure that transactions appear to be performed atomically, conflictsmust be detected and resolved. The two systems, i.e., the eager and lazyversioning systems, detect conflicts by implementing a conflictdetection policy, either optimistic or pessimistic. An optimistic systemexecutes transactions in parallel, checking for conflicts only when atransaction commits. A pessimistic system checks for conflicts at eachload and store. Similar to versioning, conflict detection also uses thecache, marking each line as either part of the read-set, part of thewrite-set, or both. The two systems resolve conflicts by implementing acontention management policy. Many contention management policies exist,some are more appropriate for optimistic conflict detection and some aremore appropriate for pessimistic. Described below are some examplepolicies.

Since each transactional memory (TM) system needs both versioningdetection and conflict detection, these options give rise to fourdistinct TM designs: Eager-Pessimistic (EP), Eager-Optimistic (EO),Lazy-Pessimistic (LP), and Lazy-Optimistic (LO). Table 2 brieflydescribes all four distinct TM designs.

FIGS. 1 and 2 depict an example of a multicore TM environment. FIG. 1shows many TM-enabled CPUs (CPU1 114 a, CPU2 114 b, etc.) on one die100, connected with an interconnect 122, under management of aninterconnect control 120 a, 120 b. Each CPU 114 a, 114 b (also known asa Processor) may have a split cache consisting of an Instruction Cache116 a, 116 b for caching instructions from memory to be executed and aData Cache 118 a, 118 b with TM support for caching data (operands) ofmemory locations to be operated on by CPU 114 a, 114 b (in FIG. 1, eachCPU 114 a, 114 b and its associated caches are referenced as 112 a, 112b). In an implementation, caches of multiple dies 100 are interconnectedto support cache coherency between the caches of the multiple dies 100.In an implementation, a single cache, rather than the split cache isemployed holding both instructions and data. In implementations, the CPUcaches are one level of caching in a hierarchical cache structure. Forexample each die 100 may employ a shared cache 124 to be shared amongstall the CPUs on the die 100. In another implementation, each die mayhave access to a shared cache 124, shared amongst all the processors ofall the dies 100.

FIG. 2 shows the details of an example transactional CPU environment112, having a CPU 114, including additions to support TM. Thetransactional CPU (processor) 114 may include hardware for supportingRegister Checkpoints 126 and special TM Registers 128. The transactionalCPU cache may have the MESI bits 130, Tags 140 and Data 142 of aconventional cache but also, for example, R bits 132 showing a line hasbeen read by the CPU 114 while executing a transaction and W bits 138showing a line has been written-to by the CPU 114 while executing atransaction.

A key detail for programmers in any TM system is how non-transactionalaccesses interact with transactions. By design, transactional accessesare screened from each other using the mechanisms above. However, theinteraction between a regular, non-transactional load with a transactioncontaining a new value for that address must still be considered. Inaddition, the interaction between a non-transactional store with atransaction that has read that address must also be explored. These areissues of the database concept isolation.

A TM system is said to implement strong isolation, sometimes calledstrong atomicity, when every non-transactional load and store acts likean atomic transaction. Therefore, non-transactional loads cannot seeuncommitted data and non-transactional stores cause atomicity violationsin any transactions that have read that address. A system where this isnot the case is said to implement weak isolation, sometimes called weakatomicity.

Strong isolation is often more desirable than weak isolation due to therelative ease of conceptualization and implementation of strongisolation. Additionally, if a programmer has forgotten to surround someshared memory references with transactions, causing bugs, then withstrong isolation, the programmer will often detect that oversight usinga simple debug interface because the programmer will see anon-transactional region causing atomicity violations. Also, programswritten in one model may work differently on another model. Further,strong isolation is often easier to support in hardware TM than weakisolation. With strong isolation, since the coherence protocol alreadymanages load and store communication between processors, transactionscan detect non-transactional loads and stores and act appropriately. Toimplement strong isolation in software Transactional Memory (TM),non-transactional code must be modified to include read- andwrite-barriers; potentially crippling performance. Although great efforthas been expended to remove many un-needed barriers, such techniques areoften complex and performance is typically far lower than that of HTMs.

TABLE 2 Transactional Memory Design Space VERSIONING Lazy Eager CONFLICTOptimistic Storing updates in a write Not practical: waiting to updateDETECTION buffer; detecting conflicts memory until commit time but atcommit time. detecting conflicts at access time guarantees wasted workand provides no advantage Pessimistic Storing updates in a Updatingmemory, keeping old writebuffer; detecting values in undo log; detectingconflicts at access time. conflicts at access time.

Table 2 illustrates the fundamental design space of transactional memory(versioning and conflict detection).

Eager-Pessimistic (EP)

This first TM design described below is known as Eager-Pessimistic. AnEP system stores its write-set “in place” (hence the name “eager”) and,to support rollback, stores the old values of overwritten lines in an“undo log”. Processors use the W 138 and R 132 cache bits to track readand write-sets and detect conflicts when receiving snooped loadrequests. Perhaps the most notable examples of EP systems in knownliterature are LogTM and UTM.

Beginning a transaction in an EP system is much like beginning atransaction in other systems: tm_begin( ) takes a register checkpoint,and initializes any status registers. An EP system also requiresinitializing the undo log, the details of which are dependent on the logformat, but often involve initializing a log base pointer to a region ofpre-allocated, thread-private memory, and clearing a log boundsregister.

Versioning: In EP, due to the way eager versioning is designed tofunction, the MESI bits 130 state transitions (cache line indicatorscorresponding to Modified, Exclusive, Shared, and Invalid code states)are left mostly unchanged. Outside of a transaction, the MESI bits 130state transitions are left completely unchanged. When reading a lineinside a transaction, the standard coherence transitions apply (S(Shared)→S, I (Invalid)→S, or I→E (Exclusive)), issuing a load miss asneeded, but the R 132 bit is also set. Likewise, writing a line appliesthe standard transitions (S→M, E→I, I→M), issuing a miss as needed, butalso sets the W 138 (Written) bit. The first time a line is written, theold version of the entire line is loaded then written to the undo log topreserve it in case the current transaction aborts. The newly writtendata is then stored “in-place,” over the old data.

Conflict Detection: Pessimistic conflict detection uses coherencemessages exchanged on misses, or upgrades, to look for conflicts betweentransactions. When a read miss occurs within a transaction, otherprocessors receive a load request; but they ignore the request if theydo not have the needed line. If the other processors have the neededline non-speculatively or have the line R 132 (Read), they downgradethat line to S, and in certain cases issue a cache-to-cache transfer ifthey have the line in MESI's bits 130 M or E state. However, if thecache has the line W 138, then a conflict is detected between the twotransactions and additional action(s) must be taken.

Similarly, when a transaction seeks to upgrade a line from shared tomodified (on a first write), the transaction issues an exclusive loadrequest, which is also used to detect conflicts. If a receiving cachehas the line non-speculatively, then the line is invalidated, and incertain cases a cache-to-cache transfer (M or E states) is issued. But,if the line is R 132 or W 138, a conflict is detected.

Validation: Because conflict detection is performed on every load, atransaction always has exclusive access to its own write-set. Therefore,validation does not require any additional work.

Commit: Since eager versioning stores the new version of data items inplace, the commit process simply clears the W 138 and R 132 bits anddiscards the undo log.

Abort: When a transaction rolls back, the original version of each cacheline in the undo log must be restored, a process called “unrolling” or“applying” the log. This is done during tm_discard( ) and must be atomicwith regard to other transactions. Specifically, the write-set muststill be used to detect conflicts: this transaction has the only correctversion of lines in its undo log, and requesting transactions must waitfor the correct version to be restored from that log. Such a log can beapplied using a hardware state machine or software abort handler.

Eager-Pessimistic has the characteristics of: Commit is simple and sinceit is in-place, very fast. Similarly, validation is a no-op. Pessimisticconflict detection detects conflicts early, thereby reducing the numberof “doomed” transactions. For example, if two transactions are involvedin a Write-After-Read dependency, then that dependency is detectedimmediately in pessimistic conflict detection. However, in optimisticconflict detection such conflicts are not detected until the writercommits.

Eager-Pessimistic also has the characteristics of: As described above,the first time a cache line is written, the old value must be written tothe log, incurring extra cache accesses. Aborts are expensive as theyrequire undoing the log. For each cache line in the log, a load must beissued, perhaps going as far as main memory before continuing to thenext line. Pessimistic conflict detection also prevents certainserializable schedules from existing.

Additionally, because conflicts are handled as they occur, there is apotential for livelock and careful contention management mechanisms mustbe employed to guarantee forward progress.

Lazy-Optimistic (LO)

Another popular TM design is Lazy-Optimistic (LO), which stores itswrite-set in a “write buffer” or “redo log” and detects conflicts atcommit time (still using the R 132 and W 138 bits).

Versioning: Just as in the EP system, the MESI protocol of the LO designis enforced outside of the transactions. Once inside a transaction,reading a line incurs the standard MESI transitions but also sets the R132 bit. Likewise, writing a line sets the W 138 bit of the line, buthandling the MESI transitions of the LO design is different from that ofthe EP design. First, with lazy versioning, the new versions of writtendata are stored in the cache hierarchy until commit while othertransactions have access to old versions available in memory or othercaches. To make available the old versions, dirty lines (M lines) mustbe evicted when first written by a transaction. Second, no upgrademisses are needed because of the optimistic conflict detection feature:if a transaction has a line in the S state, it can simply write to itand upgrade that line to an M state without communicating the changeswith other transactions because conflict detection is done at committime.

Conflict Detection and Validation: To validate a transaction and detectconflicts, LO communicates the addresses of speculatively modified linesto other transactions only when it is preparing to commit. Onvalidation, the processor sends one, potentially large, network packetcontaining all the addresses in the write-set. Data is not sent, butleft in the cache of the committer and marked dirty (M). To build thispacket without searching the cache for lines marked W, a simple bitvector is used, called a “store buffer,” with one bit per cache line totrack these speculatively modified lines. Other transactions use thisaddress packet to detect conflicts: if an address is found in the cacheand the R 132 and/or W 138 bits are set, then a conflict is initiated.If the line is found but neither R 132 nor W 138 is set, then the lineis simply invalidated, which is similar to processing an exclusive load.

To support transaction atomicity, these address packets must be handledatomically, i.e., no two address packets may exist at once with the sameaddresses. In an LO system, this can be achieved by simply acquiring aglobal commit token before sending the address packet. However, atwo-phase commit scheme could be employed by first sending out theaddress packet, collecting responses, enforcing an ordering protocol(perhaps oldest transaction first), and committing once all responsesare satisfactory.

Commit: Once validation has occurred, commit needs no special treatment:simply clear W 138 and R 132 bits and the store buffer. Thetransaction's writes are already marked dirty in the cache and othercaches' copies of these lines have been invalidated via the addresspacket. Other processors can then access the committed data through theregular coherence protocol.

Abort: Rollback is equally easy: because the write-set is containedwithin the local caches, these lines can be invalidated, then clear W138 and R 132 bits and the store buffer. The store buffer allows W linesto be found to invalidate without the need to search the cache.

Lazy-Optimistic has the characteristics of: Aborts are very fast,requiring no additional loads or stores and making only local changes.More serializable schedules can exist than found in EP, which allows anLO system to more aggressively speculate that transactions areindependent, which can yield higher performance. Finally, the latedetection of conflicts can increase the likelihood of forward progress.

Lazy-Optimistic also has the characteristics of: Validation takes globalcommunication time proportional to size of write set. Doomedtransactions can waste work since conflicts are detected only at committime.

Lazy-Pessimistic (LP)

Lazy-Pessimistic (LP) represents a third TM design option, sittingsomewhere between EP and LO: storing newly written lines in a writebuffer but detecting conflicts on a per access basis.

Versioning: Versioning is similar but not identical to that of LO:reading a line sets its R bit 132, writing a line sets its W bit 138,and a store buffer is used to track W lines in the cache. Also, dirty(M) lines must be evicted when first written by a transaction, just asin LO. However, since conflict detection is pessimistic, load exclusivesmust be performed when upgrading a transactional line from I, S→M, whichis unlike LO.

Conflict Detection: LP's conflict detection operates the same as EP's:using coherence messages to look for conflicts between transactions.

Validation: Like in EP, pessimistic conflict detection ensures that atany point, a running transaction has no conflicts with any other runningtransaction, so validation is a no-op.

Commit: Commit needs no special treatment: simply clear W 138 and R 132bits and the store buffer, like in LO.

Abort: Rollback is also like that of LO: simply invalidate the write-setusing the store buffer and clear the W and R bits and the store buffer.

Eager-Optimistic (EO)

The LP has the characteristics of: Like LO, aborts are very fast. LikeEP, the use of pessimistic conflict detection reduces the number of“doomed” transactions. Like EP, some serializable schedules are notallowed and conflict detection must be performed on each cache miss.

The final combination of versioning and conflict detection isEager-Optimistic (EO). EO may be a less than optimal choice for HTMsystems: since new transactional versions are written in-place, othertransactions have no choice but to notice conflicts as they occur (i.e.,as cache misses occur). But since EO waits until commit time to detectconflicts, those transactions become “zombies,” continuing to execute,wasting resources, yet are “doomed” to abort.

EO has proven to be useful in STMs and is implemented by Bartok-STM andMcRT. A lazy versioning STM needs to check its write buffer on each readto ensure that it is reading the most recent value. Since the writebuffer is not a hardware structure, this is expensive, hence thepreference for write-in-place eager versioning. Additionally, sincechecking for conflicts is also expensive in an STM, optimistic conflictdetection offers the advantage of performing this operation in bulk.

Contention Management

How a transaction rolls back once the system has decided to abort thattransaction has been described above, but, since a conflict involves twotransactions, the topics of which transaction should abort, how thatabort should be initiated, and when should the aborted transaction beretried need to be explored. These are topics that are addressed byContention Management (CM), a key component of transactional memory.Described below are policies regarding how the systems initiate abortsand the various established methods of managing which transactionsshould abort in a conflict.

Contention Management Policies

A Contention Management (CM) Policy is a mechanism that determines whichtransaction involved in a conflict should abort and when the abortedtransaction should be retried. For example, it is often the case thatretrying an aborted transaction immediately does not lead to the bestperformance. Conversely, employing a back-off mechanism, which delaysthe retrying of an aborted transaction, can yield better performance.STMs first grappled with finding the best contention management policiesand many of the policies outlined below were originally developed forSTMs.

CM Policies draw on a number of measures to make decisions, includingages of the transactions, size of read- and write-sets, the number ofprevious aborts, etc. The combinations of measures to make suchdecisions are endless, but certain combinations are described below,roughly in order of increasing complexity.

To establish some nomenclature, first note that in a conflict there aretwo sides: the attacker and the defender. The attacker is thetransaction requesting access to a shared memory location. Inpessimistic conflict detection, the attacker is the transaction issuingthe load or load exclusive. In optimistic, the attacker is thetransaction attempting to validate. The defender in both cases is thetransaction receiving the attacker's request.

An Aggressive CM Policy immediately and always retries either theattacker or the defender. In LO, Aggressive means that the attackeralways wins, and so Aggressive is sometimes called committer wins. Sucha policy was used for the earliest LO systems. In the case of EP,Aggressive can be either defender wins or attacker wins.

Restarting a conflicting transaction that will immediately experienceanother conflict is bound to waste work—namely interconnect bandwidthrefilling cache misses. A Polite CM Policy employs exponential backoff(but linear could also be used) before restarting conflicts. To preventstarvation, a situation where a process does not have resourcesallocated to it by the scheduler, the exponential backoff greatlyincreases the odds of transaction success after some n retries.

Another approach to conflict resolution is to randomly abort theattacker or defender (a policy called Randomized). Such a policy may becombined with a randomized backoff scheme to avoid unneeded contention.

However, making random choices, when selecting a transaction to abort,can result in aborting transactions that have completed “a lot of work”,which can waste resources. To avoid such waste, the amount of workcompleted on the transaction can be taken into account when determiningwhich transaction to abort. One measure of work could be a transaction'sage. Other methods include Oldest, Bulk TM, Size Matters, Karma, andPolka. Oldest is a simple timestamp method that aborts the youngertransaction in a conflict. Bulk TM uses this scheme. Size Matters islike Oldest but instead of transaction age, the number of read/writtenwords is used as the priority, reverting to Oldest after a fixed numberof aborts. Karma is similar, using the size of the write-set aspriority. Rollback then proceeds after backing off a fixed amount oftime. Aborted transactions keep their priorities after being aborted(hence the name Karma). Polka works like Karma but instead of backingoff a predefined amount of time, it backs off exponentially more eachtime.

Since aborting wastes work, it is logical to argue that stalling anattacker until the defender has finished their transaction would lead tobetter performance. Unfortunately, such a simple scheme easily leads todeadlock.

Deadlock avoidance techniques can be used to solve this problem. Greedyuses two rules to avoid deadlock. The first rule is, if a firsttransaction, T1, has lower priority than a second transaction, T0, or ifT1 is waiting for another transaction, then T1 aborts when conflictingwith T0. The second rule is, if T1 has higher priority than T0 and isnot waiting, then T0 waits until T1 commits, aborts, or starts waiting(in which case the first rule is applied). Greedy provides someguarantees about time bounds for executing a set of transactions. One EPdesign (LogTM) uses a CM policy similar to Greedy to achieve stallingwith conservative deadlock avoidance.

Example MESI coherency rules provide for four possible states in which acache line of a multiprocessor cache system may reside, M, E, S, and I,defined as follows:

Modified (M): The cache line is present only in the current cache, andis dirty; it has been modified from the value in main memory. The cacheis required to write the data back to main memory at some time in thefuture, before permitting any other read of the (no longer valid) mainmemory state. The write-back changes the line to the Exclusive state.

Exclusive (E): The cache line is present only in the current cache, butis clean; it matches main memory. It may be changed to the Shared stateat any time, in response to a read request. Alternatively, it may bechanged to the Modified state when writing to it.

Shared (S): Indicates that this cache line may be stored in other cachesof the machine and is “clean”; it matches the main memory. The line maybe discarded (changed to the Invalid state) at any time.

Invalid (I): Indicates that this cache line is invalid (unused).

TM coherency status indicators (R 132, W 138) may be provided for eachcache line, in addition to, or encoded in the MESI coherency bits. An R132 indicator indicates the current transaction has read from the dataof the cache line, and a W 138 indicator indicates the currenttransaction has written to the data of the cache line.

In another aspect of TM design, a system is designed using transactionalstore buffers. U.S. Pat. No. 6,349,361 titled “Methods and Apparatus forReordering and Renaming Memory References in a Multiprocessor ComputerSystem,” filed Mar. 31, 2000 and incorporated by reference herein in itsentirety, teaches a method for reordering and renaming memory referencesin a multiprocessor computer system having at least a first and a secondprocessor. The first processor has a first private cache and a firstbuffer, and the second processor has a second private cache and a secondbuffer. The method includes the steps of, for each of a plurality ofgated store requests received by the first processor to store a datum,exclusively acquiring a cache line that contains the datum by the firstprivate cache, and storing the datum in the first buffer. Upon the firstbuffer receiving a load request from the first processor to load aparticular datum, the particular datum is provided to the firstprocessor from among the data stored in the first buffer based on anin-order sequence of load and store operations. Upon the first cachereceiving a load request from the second cache for a given datum, anerror condition is indicated and a current state of at least one of theprocessors is reset to an earlier state when the load request for thegiven datum corresponds to the data stored in the first buffer.

The main implementation components of one such transactional memoryfacility are a transaction-backup register file for holdingpre-transaction GR (general register) content, a cache directory totrack the cache lines accessed during the transaction, a store cache tobuffer stores until the transaction ends, and firmware routines toperform various complex functions. In this section a detailedimplementation is described.

IBM zEnterprise EC12 Enterprise Server Embodiment

The IBM zEnterprise EC12 enterprise server introduces transactionalexecution (TX) in transactional memory, and is described in part in apaper, “Transactional Memory Architecture and Implementation for IBMSystem z” of Proceedings Pages 25-36 presented at MICRO-45, 1-5 Dec.2012, Vancouver, British Columbia, Canada, available from IEEE ComputerSociety Conference Publishing Services (CPS), which is incorporated byreference herein in its entirety.

Table 3 shows an example transaction. Transactions started with TBEGINare not assured to ever successfully complete with TEND, since they canexperience an aborting condition at every attempted execution, e.g., dueto repeating conflicts with other CPUs. This requires that the programsupport a fallback path to perform the same operationnon-transactionally, e.g., by using traditional locking schemes. Thisputs significant burden on the programming and software verificationteams, especially where the fallback path is not automatically generatedby a reliable compiler.

TABLE 3 Example Transaction Code LHI R0,0 *initialize retry count=0 loopTBEGIN *begin transaction JNZ abort *go to abort code if CC1=0 LT R1,lock *load and test the fallback lock JNZ lckbzy *branch if lock busy .. . perform operation . . . TEND *end transaction . . . . . . . . . . .. lckbzy TABORT *abort if lock busy; this *resumes after TBEGIN abort JOfallback *no retry if CC=3 AHI R0, 1 *increment retry count CIJNL R0,6,fallback *give up after 6 attempts PPA R0, TX *random delay based onretry count . . . potentially wait for lock to become free . . . J loop*jump back to retry fallback OBTAIN lock *using Compare&Swap . . .perform operation . . . RELEASE lock . . . . . . . . . . . .

The requirement of providing a fallback path for aborted TransactionExecution (TX) transactions can be onerous. Many transactions operatingon shared data structures are expected to be short, touch only a fewdistinct memory locations, and use simple instructions only. For thosetransactions, the IBM zEnterprise EC12 introduces the concept ofconstrained transactions; under normal conditions, the CPU 114 (FIG. 1)assures that constrained transactions eventually end successfully,albeit without giving a strict limit on the number of necessary retries.A constrained transaction starts with a TBEGINC instruction and endswith a regular TEND. Implementing a task as a constrained ornon-constrained transaction typically results in very comparableperformance, but constrained transactions simplify software developmentby removing the need for a fallback path. IBM's Transactional Executionarchitecture is further described in z/Architecture, Principles ofOperation, Tenth Edition, SA22-7832-09 published September 2012 fromIBM, incorporated by reference herein in its entirety.

A constrained transaction starts with the TBEGINC instruction. Atransaction initiated with TBEGINC must follow a list of programmingconstraints; otherwise the program takes a non-filterableconstraint-violation interruption. Exemplary constraints may include,but not be limited to: the transaction can execute a maximum of 32instructions, all instruction text must be within 256 consecutive bytesof memory; the transaction contains only forward-pointing relativebranches (i.e., no loops or subroutine calls); the transaction canaccess a maximum of 4 aligned octowords (an octoword is 32 bytes) ofmemory; and restriction of the instruction-set to exclude complexinstructions like decimal or floating-point operations. The constraintsare chosen such that many common operations like doubly linkedlist-insert/delete operations can be performed, including the verypowerful concept of atomic compare-and-swap targeting up to 4 alignedoctowords. At the same time, the constraints were chosen conservativelysuch that future CPU implementations can assure transaction successwithout needing to adjust the constraints, since that would otherwiselead to software incompatibility.

TBEGINC mostly behaves like XBEGIN in TSX or TBEGIN on IBM's zEC12servers, except that the floating-point register (FPR) control and theprogram interruption filtering fields do not exist and the controls areconsidered to be zero. On a transaction abort, the instruction addressis set back directly to the TBEGINC instead of to the instruction after,reflecting the immediate retry and absence of an abort path forconstrained transactions.

Nested transactions are not allowed within constrained transactions, butif a TBEGINC occurs within a non-constrained transaction it is treatedas opening a new non-constrained nesting level just like TBEGIN would.This can occur, e.g., if a non-constrained transaction calls asubroutine that uses a constrained transaction internally. Sinceinterruption filtering is implicitly off, all exceptions during aconstrained transaction lead to an interruption into the operatingsystem (OS). Eventual successful finishing of the transaction relies onthe capability of the OS to page-in the at most 4 pages touched by anyconstrained transaction. The OS must also ensure time-slices long enoughto allow the transaction to complete.

TABLE 4 Transaction Code Example TBEGINC *begin constrained transaction. . . perform operation . . . TEND   *end transaction

Table 4 shows the constrained-transactional implementation of the codein Table 3, assuming that the constrained transactions do not interactwith other locking-based code. No lock testing is shown therefore, butcould be added if constrained transactions and lock-based code weremixed.

When failure occurs repeatedly, software emulation is performed usingmillicode as part of system firmware. Advantageously, constrainedtransactions have desirable properties because of the burden removedfrom programmers.

With reference to FIG. 3, the IBM zEnterprise EC12 processor introducedthe transactional execution facility. The processor can decode 3instructions per clock cycle; simple instructions are dispatched assingle micro-ops, and more complex instructions are cracked intomultiple micro-ops. The micro-ops (Uops 232 b) are written into aunified issue queue 216, from where they can be issued out-of-order. Upto two fixed-point, one floating-point, two load/store, and two branchinstructions can execute every cycle. A Global Completion Table (GCT)232 holds every micro-op 232 b and a transaction nesting depth (TND) 232a. The GCT 232 is written in-order at decode time, tracks the executionstatus of each micro-op 232 b, and completes instructions when allmicro-ops 232 b of the oldest instruction group have successfullyexecuted.

The level 1 (L1) data cache 240 is a 96 KB (kilo-byte) 6-way associativecache with 256 byte cache-lines and 4 cycle use latency, coupled to aprivate 1 MB (mega-byte) 8-way associative 2nd-level (L2) data cache 268with 7 cycles use-latency penalty for L1 240 misses. The L1 240 cache isthe cache closest to a processor and Ln cache is a cache at the nthlevel of caching. Both L1 240 and L2 268 caches are store-through. Sixcores on each central processor (CP) chip share a 48 MB 3rd-levelstore-in cache, and six CP chips are connected to an off-chip 384 MB4th-level cache, packaged together on a glass ceramic multi-chip module(MCM). Up to 4 multi-chip modules (MCMs) can be connected to a coherentsymmetric multi-processor (SMP) system with up to 144 cores (not allcores are available to run customer workload).

Coherency is managed with a variant of the MESI protocol. Cache-linescan be owned read-only (shared) or exclusive; the L1 240 and L2 268 arestore-through and thus do not contain dirty lines. The L3 272 and L4caches (not shown) are store-in and track dirty states. Each cache isinclusive of all its connected lower level caches.

Coherency requests are called “cross interrogates” (XI) and are senthierarchically from higher level to lower-level caches, and between theL4s. When one core misses the L1 240 and L2 268 and requests the cacheline from its local L3 272, the L3 272 checks whether it owns the line,and if necessary sends an XI to the currently owning L2 268/L1 240 underthat L3 272 to ensure coherency, before it returns the cache line to therequestor. If the request also misses the L3 272, the L3 272 sends arequest to the L4 (not shown), which enforces coherency by sending XIsto all necessary L3s under that L4, and to the neighboring L4s. Then theL4 responds to the requesting L3 which forwards the response to the L2268/L1 240.

Note that due to the inclusivity rule of the cache hierarchy, sometimescache lines are XI′ed from lower-level caches due to evictions onhigher-level caches caused by associativity overflows from requests toother cache lines. These XIs can be called “LRU XIs”, where LRU standsfor least recently used.

Making reference to yet another type of XI requests, Demote-XIstransition cache-ownership from exclusive into read-only state, andExclusive-XIs transition cache ownership from exclusive into invalidstate. Demote-XIs and Exclusive-XIs need a response back to the XIsender. The target cache can “accept” the XI, or send a “reject”response if it first needs to evict dirty data before accepting the XI.The L1 240/L2 268 caches are store through, but may reject demote-XIsand exclusive XIs if they have stores in their store queues that need tobe sent to L3 before downgrading the exclusive state. A rejected XI willbe repeated by the sender. Read-only-XIs are sent to caches that own theline read-only; no response is needed for such XIs since they cannot berejected. The details of the SMP protocol are similar to those describedfor the IBM z10 by P. Mak, C. Walters, and G. Strait, in “IBM System z10processor cache subsystem microarchitecture”, IBM Journal of Researchand Development, Vol 53:1, 2009, which is incorporated by referenceherein in its entirety.

Transactional Instruction Execution

FIG. 3 depicts example components of an example transactional executionenvironment, including a CPU and caches/components with which itinteracts (such as those depicted in FIGS. 1 and 2). The instructiondecode unit 208 (IDU) keeps track of the current transaction nestingdepth 212 (TND). When the IDU 208 receives a TBEGIN instruction, thenesting depth 212 is incremented, and conversely decremented on TENDinstructions. The nesting depth 212 is written into the GCT 232 forevery dispatched instruction. When a TBEGIN or TEND is decoded on aspeculative path that later gets flushed, the IDU's 208 nesting depth212 is refreshed from the youngest GCT 232 entry that is not flushed.The transactional state is also written into the issue queue 216 forconsumption by the execution units, mostly by the Load/Store Unit (LSU)280, which also has an effective address calculator 236 is included inthe LSU 280. The TBEGIN instruction may specify a transaction diagnosticblock (TDB) for recording status information, should the transactionabort before reaching a TEND instruction.

Similar to the nesting depth, the IDU 208/GCT 232 collaboratively trackthe access register/floating-point register (AR/FPR) modification masksthrough the transaction nest; the IDU 208 can place an abort requestinto the GCT 232 when an AR/FPR-modifying instruction is decoded and themodification mask blocks that. When the instruction becomesnext-to-complete, completion is blocked and the transaction aborts.Other restricted instructions are handled similarly, including TBEGIN ifdecoded while in a constrained transaction, or exceeding the maximumnesting depth.

An outermost TBEGIN is cracked into multiple micro-ops depending on theGR-Save-Mask; each micro-op 232 b (including, for example uop 0, uop 1,and uop2) will be executed by one of the two fixed point units (FXUs)220 to save a pair of GRs 228 into a special transaction-backup registerfile 224, that is used to later restore the GR 228 content in case of atransaction abort. Also the TBEGIN spawns micro-ops 232 b to perform anaccessibility test for the TDB if one is specified; the address is savedin a special purpose register for later usage in the abort case. At thedecoding of an outermost TBEGIN, the instruction address and theinstruction text of the TBEGIN are also saved in special purposeregisters for a potential abort processing later on.

TEND and NTSTG are single micro-op 232 b instructions; NTSTG(non-transactional store) is handled like a normal store except that itis marked as non-transactional in the issue queue 216 so that the LSU280 can treat it appropriately. TEND is a no-op at execution time, theending of the transaction is performed when TEND completes.

As mentioned, instructions that are within a transaction are marked assuch in the issue queue 216, but otherwise execute mostly unchanged; theLSU 280 performs isolation tracking as described in the next section.

Since decoding is in-order, and since the IDU 208 keeps track of thecurrent transactional state and writes it into the issue queue 216 alongwith every instruction from the transaction, execution of TBEGIN, TEND,and instructions before, within, and after the transaction can beperformed out-of order. It is even possible (though unlikely) that TENDis executed first, then the entire transaction, and lastly the TBEGINexecutes. Program order is restored through the GCT 232 at completiontime. The length of transactions is not limited by the size of the GCT232, since general purpose registers (GRs) 228 can be restored from thebackup register file 224.

During execution, the program event recording (PER) events are filteredbased on the Event Suppression Control, and a PER TEND event is detectedif enabled. Similarly, while in transactional mode, a pseudo-randomgenerator may be causing the random aborts as enabled by the TransactionDiagnostics Control.

Tracking for Transactional Isolation

The Load/Store Unit 280 tracks cache lines that were accessed duringtransactional execution, and triggers an abort if an XI from another CPU(or an LRU-XI) conflicts with the footprint. If the conflicting XI is anexclusive or demote XI, the LSU 280 rejects the XI back to the L3 272 inthe hope of finishing the transaction before the L3 272 repeats the XI.This “stiff-arming” is very efficient in highly contended transactions.In order to prevent hangs when two CPUs stiff-arm each other, aXI-reject counter is implemented, which triggers a transaction abortwhen a threshold is met.

The L1 cache directory 240 is traditionally implemented with staticrandom access memories (SRAMs). For the transactional memoryimplementation, the valid bits 244 (64 rows×6 ways) of the directoryhave been moved into normal logic latches, and are supplemented with twomore bits per cache line: the TX-read 248 and TX-dirty 252 bits.

The TX-read 248 bits are reset when a new outermost TBEGIN is decoded(which is interlocked against a prior still pending transaction). TheTX-read 248 bit is set at execution time by every load instruction thatis marked “transactional” in the issue queue. Note that this can lead toover-marking if speculative loads are executed, for example on amispredicted branch path. The alternative of setting the TX-read 248 bitat load completion time was too expensive for silicon area, sincemultiple loads can complete at the same time, requiring many read-portson the load-queue.

Stores execute the same way as in non-transactional mode, but atransaction mark is placed in the store queue (STQ) 260 entry of thestore instruction. At write-back time, when the data from the STQ 260 iswritten into the L1 240, the TX-dirty bit 252 in the L1 cache directory240 is set for the written cache line. Store write-back into the L1 240occurs only after the store instruction has completed, and at most onestore is written back per cycle. Before completion and write-back, loadscan access the data from the STQ 260 by means of store-forwarding; afterwrite-back, the CPU 114 (FIG. 1) can access the speculatively updateddata in the L1 240. If the transaction ends successfully, the TX-dirtybits 252 of all cache-lines are cleared, and also the TX-marks of notyet written stores are cleared in the STQ 260, effectively turning thepending stores into normal stores.

On a transaction abort, all pending transactional stores are invalidatedfrom the STQ 260, even those already completed. All cache lines thatwere modified by the transaction in the L1 240, that is, have theTX-dirty bit 252 on, have their valid bits turned off, effectivelyremoving them from the L1 240 cache instantaneously.

The architecture requires that before completing a new instruction, theisolation of the transaction read- and write-set is maintained. Thisisolation is ensured by stalling instruction completion at appropriatetimes when XIs are pending; speculative out-of order execution isallowed, optimistically assuming that the pending XIs are to differentaddresses and not actually cause a transaction conflict. This designfits very naturally with the XI-vs-completion interlocks that areimplemented on prior systems to ensure the strong memory ordering thatthe architecture requires.

When the L1 240 receives an XI, L1 240 accesses the directory to checkvalidity of the XI′ed address in the L1 240, and if the TX-read bit 248is active on the XI′ed line and the XI is not rejected, the LSU 280triggers an abort. When a cache line with active TX-read bit 248 isLRU′ed from the L1 240, a special LRU-extension vector remembers foreach of the 64 rows of the L1 240 that a TX-read line existed on thatrow. Since no precise address tracking exists for the LRU extensions,any non-rejected XI that hits a valid extension row the LSU 280 triggersan abort. Providing the LRU-extension effectively increases the readfootprint capability from the L1-size to the L2-size and associativity,provided no conflicts with other CPUs 114 (FIGS. 1 and 2) against thenon-precise LRU-extension tracking causes aborts.

The store footprint is limited by the store cache size (the store cacheis discussed in more detail below) and thus implicitly by the L2 268size and associativity. No LRU-extension action needs to be performedwhen a TX-dirty 252 cache line is LRU′ed from the L1 240.

Store Cache

In prior systems, since the L1 240 and L2 268 are store-through caches,every store instruction causes an L3 272 store access; with now 6 coresper L3 272 and further improved performance of each core, the store ratefor the L3 272 (and to a lesser extent for the L2 268) becomesproblematic for certain workloads. In order to avoid store queuingdelays, a gathering store cache 264 had to be added, that combinesstores to neighboring addresses before sending them to the L3 272.

For transactional memory performance, it is acceptable to invalidateevery TX-dirty 252 cache line from the L1 240 on transaction aborts,because the L2 268 cache is very close (7 cycles L1 240 miss penalty) tobring back the clean lines. However, it would be unacceptable forperformance (and silicon area for tracking) to have transactional storeswrite the L2 268 before the transaction ends and then invalidate alldirty L2 268 cache lines on abort (or even worse on the shared L3 272).

The two problems of store bandwidth and transactional memory storehandling can both be addressed with the gathering store cache 264. Thegathering store cache 264 is a circular queue of 64 entries, each entryholding 128 bytes of data with byte-precise valid bits. Innon-transactional operation, when a store is received from the LSU 280,the store cache 264 checks whether an entry exists for the same address,and if so gathers the new store into the existing entry. If no entryexists, a new entry is written into the queue, and if the number of freeentries falls under a threshold, the oldest entries are written back tothe L2 268 and L3 272 caches.

When a new outermost transaction begins, all existing entries in thestore cache are marked closed so that no new stores can be gathered intothem, and eviction of those entries to L2 268 and L3 272 is started.From that point on, the transactional stores coming out of the LSU 280STQ 260 allocate new entries, or gather into existing transactionalentries. The write-back of those stores into L2 268 and L3 272 isblocked, until the transaction ends successfully; at that pointsubsequent (post-transaction) stores can continue to gather intoexisting entries, until the next transaction closes those entries again.

The store cache 264 is queried on every exclusive or demote XI, andcauses an XI reject if the XI compares to any active entry. If the coreis not completing further instructions while continuously rejecting XIs,the transaction is aborted at a certain threshold to avoid hangs.

The LSU 280 requests a transaction abort when the store cache 264overflows. The LSU 280 detects this condition when it tries to send anew store that cannot merge into an existing entry, and the entire storecache 264 is filled with stores from the current transaction. The storecache 264 is managed as a subset of the L2 268: while transactionallydirty lines can be evicted from the L1 240, they have to stay residentin the L2 268 throughout the transaction. The maximum store footprint isthus limited to the store cache size of 64×128 bytes, and it is alsolimited by the associativity of the L2 268. Since the L2 268 is 8-wayassociative and has 512 rows, it is typically large enough to not causetransaction aborts.

If a transaction aborts, the store cache 264 is notified and all entriesholding transactional data are invalidated. The store cache 264 also hasa mark per doubleword (8 bytes) whether the entry was written by a NTSTGinstruction—those doublewords stay valid across transaction aborts.

Millicode-Implemented Functions

Traditionally, IBM mainframe server processors contain a layer offirmware called millicode which performs complex functions like certainCISC instruction executions, interruption handling, systemsynchronization, and RAS. Millicode includes machine dependentinstructions as well as instructions of the instruction set architecture(ISA) that are fetched and executed from memory similarly toinstructions of application programs and the operating system (OS).Firmware resides in a restricted area of main memory that customerprograms cannot access. When hardware detects a situation that needs toinvoke millicode, the instruction fetching unit 204 switches into“millicode mode” and starts fetching at the appropriate location in themillicode memory area. Millicode may be fetched and executed in the sameway as instructions of the instruction set architecture (ISA), and mayinclude ISA instructions.

For transactional memory, millicode is involved in various complexsituations. Every transaction abort invokes a dedicated millicodesub-routine to perform the necessary abort steps. The transaction-abortmillicode starts by reading special-purpose registers (SPRs) holding thehardware internal abort reason, potential exception reasons, and theaborted instruction address, which millicode then uses to store a TDB ifone is specified. The TBEGIN instruction text is loaded from an SPR toobtain the GR-save-mask, which is needed for millicode to know which GRs238 to restore.

The CPU 114 (FIG. 1) supports a special millicode-only instruction toread out the backup-GRs 224 and copy them into the main GRs 228. TheTBEGIN instruction address is also loaded from an SPR to set the newinstruction address in the PSW to continue execution after the TBEGINonce the millicode abort sub-routine finishes. That PSW may later besaved as program-old PSW in case the abort is caused by a non-filteredprogram interruption.

The TABORT instruction may be millicode implemented; when the IDU 208decodes TABORT, it instructs the instruction fetch unit to branch intoTABORT's millicode, from which millicode branches into the common abortsub-routine.

The Extract Transaction Nesting Depth (ETND) instruction may also bemillicoded, since it is not performance critical; millicode loads thecurrent nesting depth out of a special hardware register and places itinto a GR 228. The PPA instruction is millicoded; it performs theoptimal delay based on the current abort count provided by software asan operand to PPA, and also based on other hardware internal state.

For constrained transactions, millicode may keep track of the number ofaborts. The counter is reset to 0 on successful TEND completion, or ifan interruption into the OS occurs (since it is not known if or when theOS will return to the program). Depending on the current abort count,millicode can invoke certain mechanisms to improve the chance of successfor the subsequent transaction retry. The mechanisms involve, forexample, successively increasing random delays between retries, andreducing the amount of speculative execution to avoid encounteringaborts caused by speculative accesses to data that the transaction isnot actually using. As a last resort, millicode can broadcast to otherCPUs 114 (FIG. 1) to stop all conflicting work, retry the localtransaction, before releasing the other CPUs 114 to continue normalprocessing. Multiple CPUs 114 must be coordinated to not causedeadlocks, so some serialization between millicode instances ondifferent CPUs 114 is required.

A technique to make groups of store instructions atomic is the use oftransactional memory (TM). The most difficult challenge to a hardware TMis handling writes. A TM system must track stores and assemble awrite-set (WS) during the transaction. The actual WS data must bebuffered until the transaction completes successfully. In the case of asuccess, all the stores in the WS become globally visible in an atomicfashion (i.e., all at once), typically by writing the buffered data to acache. Alternatively, if the transaction aborts, then the bufferedstores are discarded, without modifying memory.

Loads are somewhat simpler, because they do not alter memory. The TMmust track transactional loads, creating a read-set (RS) that containsthe data and the access address of each transactional load. A successfultransaction transfers the data of the loads in the RS to the registerfile.

A primary function performed by transaction execution is to determineinterference by remote requests, i.e., memory accesses not correspondingto the present application thread.

Often a TM monitors conflicts with the read-set and write-set at a cacheline (e.g., a cache line that contains 64B) address granularity during atransaction. That is, the RS and the WS contain the addresses of thecache lines that transactional data reside in, not the addresses of thedata within a cache line. An RS conflict occurs if any data in a cacheline in the read-set is written to by another thread. A WS conflictoccurs if any data in a cache line in the WS is read or written byanother thread. When a conflict occurs, the transaction aborts andsquashes the effects of (i.e., undo the effects of) any loads and storesparticipating in the transaction that have affected a state of thecomputer. However, to accomplish this, an implementation of a TM doesnot allow any loads and stores that participate in a transaction toalter the architected state of a computer until the transactionsuccessfully completes.

To improve performance, a computer architecture often includes prefetchinstructions that can move data from main memory or a lower cache levelto a higher cache level (closer to the processor) in anticipation of anaccess to the data. The execution of a prefetch instruction can bespeculative in nature in that it may be performed before it is knownthat the data moved by the prefetch instruction will actually beaccessed. Data in anticipation of either a read access or a write accessmay be prefetched, which is called a read prefetch or a write prefetchrespectively. Cache coherency protocols in computers, that enable thesharing, synchronization, and parallel processing of data, often requirea processor to gain ownership of the data before it can be written to(i.e., changed). Because the data may be currently owned by anotherprocessor, gaining ownership can be a lengthy process. The concept ofownership is necessary to prevent data that is being read by oneprocessor from a memory location from being changed by another processorthat writes to the same location or a different location (if copies ofthe data are in different locations). Therefore, it is oftenadvantageous for a processor to execute a write prefetch instruction togain ownership of the data in anticipation of writing to the data sothat processing is not delayed while ownership is acquired.

In accordance with the present invention, embodiments avoid performancedegradation traditionally associated with the execution of prefetchinstructions in existing transactional memory systems, where prefetchinstructions (i.e., prefetch requests) cause transactions to fail. In anembodiment of the present invention, the execution of prefetchinstructions does not require a memory transaction in a transactionalmemory system in a local processor to abort due to an address conflictwith a prefetch request from a remote processor. In an embodiment of thepresent invention, a prefetch instruction (i.e., a prefetch request) cancause a memory transaction (i.e., a transaction) to abort due to anaddress conflict under certain conditions (e.g., if a priority orimportance of the prefetch is higher than that of the memorytransaction). In an embodiment, a prefetch request that conflicts with atransaction is evaluated, where evaluated includes one or more of i)comparing a priority of the prefetch request with a priority of thetransaction, ii) comparing a priority of the local processor with apriority of the remote processor, and iii) determining an importance ofthe prefetch request. In an embodiment, an action is performed that isbased on the evaluation where the action includes one or more of i)aborting the prefetch request, ii) quiescing the prefetch request (i.e.,placing the prefetch request in a dormant but watchful state whilewaiting for a condition to occur before an action is taken), iii)delaying the prefetch request for a delay period (i.e., placing theprefetch request in a dormant state until the delay period is overbefore taking an action), and iv) executing the prefetch request. In anembodiment, a duration of a given delay is determined by one or both ofi) one or more conditions in the local processor, and ii) one or moreconditions in the remote processor. For example, in an embodiment, adelay period, i.e., a duration of a given delay, depends on conditionsin the local processor (e.g., cpu utilization, cache utilization, andnumber of queued workloads, and current instructions executed percycle). In another example, in one embodiment, a delay period depends onconditions in a remote processor that generated the prefetch request(e.g., cpu utilization, cache utilization, and number of queuedworkloads, and current instructions executed per cycle).

In an embodiment, a prefetch request is suppressed if the prefetchrequest conflicts with a transaction, where the meaning of the termsuppressed includes any temporal sequence of and combination of: i) notimmediately processing the prefetch request (i.e., delaying the prefetchrequest), ii) queuing the prefetch request, and iii) aborting theprefetch request. Each of these operations constitute a suppression of aprefetch request in the present invention.

In an embodiment, a prefetch request operation (i.e., a prefetchoperation) is a memory access that is performed during the execution ofa prefetch request. It is likely that a performance increase due toexecuting a prefetch request is considerably less than a performancedecrease that results from a memory transaction abort. In an embodiment,a prefetch request participates in a prefetch protocol (which is part ofthe memory and cache coherence protocols described herein) thatidentifies a read or a write memory access as a prefetch request andprovides information about the prefetch request such that the prefetchrequest can be managed without causing a transaction abort if the memoryoperation associated with the prefetch request conflicts with thetransaction. In an embodiment, a prefetch protocol includes a readprefetch request operation and a write prefetch request operation. In anembodiment, a prefetch request includes one or more prefetch operations.In an embodiment, the prefetch protocol includes one or more prefetchacknowledgements sent from a receiver of a prefetch request to thesource of the prefetch request (i.e., notifying the source of a prefetchrequest of the status of the prefetch request). In an embodiment, aprefetch acknowledgement includes information about the disposition(i.e., the status) of the prefetch at the receiver of the prefetchrequest (e.g., a local processor).

In an embodiment, the prefetch request is discarded in the receivingnode if it is determined that processing the prefetch request wouldcause a memory transaction abort. In an embodiment, if the source of theprefetch request (e.g., a remote processor) designates a prefetchrequest as being particularly important, the prefetch request is queuedin the receiving node and performed when it will no longer cause amemory transaction abort. In an embodiment, there is a separate queuefor each remote processor and a prefetch request from a remote processoris queued in the queue for that remote processor. In an embodiment,there is a queue for prefetch requests for data for exclusive ownershipand a queue for prefetch requests for data for shared ownership. In anembodiment, if the prefetch request contains a plurality of prefetchrequest operations (i.e., prefetch operations), then each prefetchoperation is queued separately. In an embodiment, a priority isassociated with a prefetch request (e.g., the priority of the source ofthe prefetch). In an embodiment, a prefetch operation in a queue in thereceiver of the prefetch request is serviced according to the priorityof the prefetch request. In an embodiment, a priority is associated withthe source of a prefetch request and the receiver of a prefetch request.In an embodiment, if the source of a prefetch request (e.g., a remoteprocessor) has a higher priority than the priority of the receiver ofthe prefetch (e.g., a local processor), the prefetch request is servicedand is allowed to cause a transaction to abort in the receiver if itconflicts with the transaction. In an embodiment, there is a separatequeue for each prefetch request priority and a prefect with a priorityis queued in the queue for that priority.

In an embodiment, a prefetch protocol includes a prefetch time horizonthat is associated with each prefetch request that is measured in cyclesthat the receiver of a prefetch request uses to decide how to manage aprefetch request. In an embodiment, a prefetch request is discardedafter a prefetch time horizon associated with the prefetch requesttranspires if it is queued and has not yet been serviced. In anembodiment, if a prefetch request is discarded by the receiver of theprefetch request, then a notification is sent to the source of theprefetch request that the prefetch request was discarded. In someembodiments, the time horizon is specified in other metrics, e.g.,number of instructions executed, number of memory instructions executed,or any other metric associated to a program and/or processor.

In an embodiment, a prefetch protocol includes a specification of aprefetch time horizon that is associated with a prefetch operation thatis divided into time steps, from the near future to the far future, witheach time step specified in a preset number of cycles that the receiverof a prefetch request uses to manage a prefetch request. In anembodiment, an execution of a prefetch request is delayed for a numberof time steps that is passed to a receiver of the prefetch request bythe prefetch protocol in a delay information. In an embodiment, thereceiver of the prefetch request delays the prefetch request for thespecified number of time steps (i.e., a delay period, or equivalently, adelay duration), after which, an attempt is made to execute the prefetchrequest. In an embodiment, if the prefetch request encounters a conflictwith a memory transaction in the attempt to execute the prefetch requestafter the delay transpires, the prefetch request is aborted. In anembodiment, if the prefetch request encounters a conflict with a memorytransaction in the attempt to execute the prefetch request after thedelay transpires, the prefetch request is queued in a queue.

In an embodiment, if a prefetch request has a conflict with a memorytransaction and the number of time steps specified in the prefetchprotocol is less than a threshold, then the prefetch request isdiscarded; otherwise the prefetch request is queued in a queue. In anembodiment, a prefetch request in a prefetch protocol is designated as ashort-range prefetch request or a long-range prefetch request. In anembodiment, a short-range prefetch request is alive (i.e., the prefetchrequest has a potential to be executed) for a specified number ofcycles. While a short-range prefetch request is alive, a short-rangeprefetch request is queued if the short-range prefetch request has aconflict with a memory transaction, and when the short-range prefetchrequest is no longer alive, it is aborted. In an embodiment, along-range prefetch request remains alive until it is executed. In anembodiment, a long-range prefetch request is queued in a queue if it hasa conflict with a memory transaction, and is removed from the queue andexecuted when that long-range prefetch request no longer has a conflictwith a memory transaction. In an embodiment, a long-range prefetchrequest is removed from the queue and discarded if the queue becomesfull and that long-range prefetch request is the oldest prefetch requestin the queue. In an embodiment, if there is at least one entry in thequeue that is not a long-range prefetch request and a given long-rangeprefetch request in the queue is the oldest prefetch request in thequeue, then the oldest long-range prefetch request is not removed fromthe queue and discarded if the queue becomes full. In one suchembodiment and scenario, if the queue becomes full and at least oneentry is not a long-range prefetch request, then at least one entry inthe queue that is not a long-range prefetch request is removed from thequeue. In an embodiment, a long-range prefetch request is not removedfrom the queue and discarded if the queue becomes full.

FIG. 4 depicts processor system 1101 that, in an embodiment, includesprocessor A 1102 connected to cache system A 1103 and processor B 1106connected to cache system B 1107. Cache system A 1103 and cache system B1107 are connected to system bus 1111 and are essentially similar. Cachesystem A 1103 includes transactional memory system A 1104 and caches A1105. Cache system B 1107 includes transactional memory system B 1108and caches B 1109. System memory 1110 is attached to system bus 1110. Inan embodiment, cache system A 1103 and cache system B 1107 participatein a cache coherency protocol that enables processor A 1102 andprocessor B to execute instructions that operate on data that is sharedby both processors in an organized manner. For example, processor B 1106can execute a read instruction that accesses data from cache system B1107 and if cache system B 1107 does not have the data, cache system B1107 will assert a cache coherency protocol request for the data onsystem bus 1111. Cache system A 1103 monitors (snoops) cache coherencyprotocol requests on system bus 1111, and if cache system A 1103 has therequested data, it will assert the data on system bus 1111. Cache systemA 1103 also compares the address of the requested data to determine ifthe address conflicts with an active transactional memory operation. Inan embodiment, transactional memory system A 1104 in cache system A 1103determines if a memory access address conflicts with that of an activetransactional memory operation.

FIG. 5 shows transactional memory system A 1104 in more detail. In anembodiment, transactional memory system A 1104 includes write sets 2205,read sets 2206, abort manager 227, and prefetch manager 2208. Write sets2205 contains the target addresses of writes (stores) that areparticipating in a transaction. Read sets 2206 contains the targetaddresses of reads (loads) that are participating in a transaction. Inan embodiment, read sets 2206 and write sets 2205 contain the read andwrite addresses respectively of one or more memory rad and writeaccesses within a transaction.

In an embodiment, inputs to transactional memory system A 1104 includenon-prefetch operations (i.e., non-prefetch memory accesses) onnon-prefetch operations bus 2210 and prefetch operations (i.e., prefetchrequest memory accesses) on prefetch operations bus 2209. Address decode2211 decodes the address of a non-prefetch operation and asserts thetarget address of a non-prefetch read operation on read address bus 2201and the target address of a non-prefetch write operation on writeaddress bus 2202. Address decode 2212 decodes the address of a prefetchoperation and asserts the target address of a prefetch read operation ison prefetch read address bus 2203 and the target address of a prefetchwrite operation on prefetch write address bus 2204. Prefetch manager2208 is connected to abort manager 2207 and receives all prefetchoperations that are input to transactional memory system A 1104 fromprefetch operations bus 2209.

Read address bus 2201 and write address bus 2202 are inputs to abortmanager 2207. In an embodiment, if abort manager 2207 detects that anaddress on read address bus 2201 matches an address in write set 2205,abort manager 2207 aborts the transactional memory operation thatcontains writes to the address. In an embodiment, if abort manager 2207detects that an address on write address bus 2202 matches an address inwrite set 2205 or matches an address in read set 2206, abort manager2207 aborts the transaction.

Prefetch read address bus 2203 and prefetch write address bus 2204 areinputs to abort manager 2207. In an embodiment, if abort manager 2207detects that an address on prefetch read address bus 2203 matches anaddress in write set 2205, abort manager 2207 notifies prefetch manager2208 that a conflict with a current prefetch operation address hasoccurred. In an embodiment, abort manager 2207 does not abort thetransaction that contains a write to the address. In an embodiment,abort manager 2207 aborts the transaction that contains a write to theaddress if directed to abort by prefetch manager 2208. In an embodiment,if abort manager 2207 detects that an address on prefetch write addressbus 2204 matches an address in write set 2205 or matches an address inread set 2206, abort manager 2207 notifies prefetch manager 2208 that aconflict with a current prefetch operation address has occurred. In anembodiment, abort manager 2207 does not abort the transaction. In anembodiment, abort manager 2207 aborts the transaction if directed to doso by prefetch manager 2208.

In an embodiment, prefetch manager 2208 includes prefetch queue 2213,queue manager 2214, and local-remote prefetch 2216. In an embodiment,local-remote prefetch 2216 classifies a prefetch operation as either alocal prefetch operation (e.g., from a local processor) or a remoteprefetch operation (e.g., from a remote processor). A prefetch operationis either a local prefetch operation or a remote prefetch operationrelative to a memory transaction. A prefetch operation is local to amemory transaction if the prefetch operation is generated on the sameprocessor (e.g., the local processor) that generated the memorytransaction and accesses a cache that is accessed by the memorytransaction; otherwise the prefetch operation is a remote prefetchoperation (e.g., from a remote processor) relative to the memorytransaction. Therefore, a prefetch operation is local to alltransactional memory operations created by the thread of execution thatcreated the prefetch operation. In an embodiment, a prefetch operationthat local-remote prefetch 2216 classifies as local relative to atransaction cannot conflict with reads or writes in write sets 2205 orread sets 2206 that are associated with the transaction. In particular,a prefetch is local when it is attached to the present transactionalmemory system (e.g., when multiple threads within a single processor,e.g., processor 1102, or multiple processors are attached to the presenttransactional memory system).

In an embodiment, if prefetch manager 2208 is notified that a currentprefetch operation conflicts with a transaction, prefetch manager 2208discards the prefetch operation and the prefetch request associated withthe prefetch operation. In an embodiment, if prefetch manager 2208discards a prefetch operation, prefetch manager 2208 sends a response tothe source of the prefetch notifying the source that the prefetch wasdiscarded. In an embodiment, if prefetch manager 2208 is notified byabort manager 2207 that a current prefetch operation conflicts with anexisting transaction, queue manager 2214 queues the prefetch operationin prefetch queue 2213. In an embodiment, prefetch manager 2208processes the queued prefetch operation after the conflictingtransactional memory operation is either complete or is aborted. In anembodiment, if queue manager 2214 queues a prefetch operation inprefetch queue 2213, prefetch manager 2208 sends a response to thesource of the prefetch request notifying the source that the prefetchoperation was queued.

In an embodiment, prefetch operations on prefetch operations bus 2209participate in a prefetch protocol that includes a prefetch request. Aprefetch request describes a memory access for data that is a prefetchmemory access and not a regular read or write memory access for whichthe need for the data is certain. In an embodiment, a prefetch requestis identified with a unique tag. In an embodiment, the unique tag maynot be reused in a system until such time that it becomes impossible fora duplicate of the unique tag to exist in the system. In an embodiment,the prefetch request includes a prefetch information (included in theprefetch request at the source of the prefetch request) that enables theprefetch operation (that is the result of the prefetch request) to beefficiently managed by prefetch manager 2208 when the prefetch operationis processed. In an embodiment, the prefetch information includes apriority information about the source of the prefetch operation. In anembodiment, if prefetch manager finds that the priority of the source ofthe prefetch operation exceeds the priority of the process that createda transactional memory operation (that has an address conflict with theprefetch operation), prefetch manager 2208 allows the prefetch operationto proceed and the transaction is aborted by abort manager 2207. In anembodiment, prefetch manager finds that the ratio of the priority of thesource of the prefetch operation to the priority of the process thatcreated a transaction (that has an address conflict with the prefetchoperation) exceeds a threshold, the prefetch operation proceeds and thetransaction is aborted. In an embodiment, if a prefetch operation causesa memory transaction to abort, prefetch manager 2208 sends anotification to the source of the prefetch operation indicating that theprefetch operation proceeded with a consequential abort of atransaction. In addition, a transaction abort is communicated to theprocessor corresponding to the transaction to roll back the transactionstate, and restart executing the transaction, or an alternate path.

In an embodiment, the prefetch information in a prefetch requestincludes a prefetch horizon. In an embodiment, the prefetch horizon isspecified as a number of cycles for which queue manager 2214 can queuethe prefetch operation in prefetch queue 2213 if it is in addressconflict with a transactional memory operation. In an embodiment, theprefetch horizon is specified as a number of instruction executions atthe source of the prefetch, which is converted into a number of cyclesat the receiver of the prefetch. In an embodiment, a conversion from thenumber of instruction executions at the source to a number of cycles atthe receiver is performed using a static conversion factor, which doesnot change during execution. In an embodiment, a conversion from thenumber of instruction executions at the source to a number of cycles atthe receiver is performed during execution using a dynamically computedconversion factor (e.g., by multiplying the number of instructions by acurrent cycles-per-instruction conversion factor) computed by thereceiver of the prefetch.

In an embodiment, a local processor receiving prefetch requests tracksthe number of cycles expected to be remaining in the one or more currenttransactions with which the prefetch conflicts. In an embodiment, acount of the expected number of remaining cycles in one or moretransactions is maintained. In an embodiment, if the end time of a timehorizon for a prefetch is less than the end time after an expectednumber of cycles that remains in a conflicting transaction, the prefetchrequest is discarded and otherwise the prefetch request is queued.

In an embodiment, if the prefetch operation has not been processedwithin the prefetch horizon number of cycles, queue manager 2214 removesthe prefetch operation from prefetch queue 2213 and discards it. In anembodiment, if the prefetch operation is discarded after the prefetchhorizon number of cycles, prefetch manager 2208 notifies the source ofthe prefetch that the prefetch has been discarded because the prefetchhorizon was exceeded.

In an embodiment, the prefetch information in a prefetch requestincludes the importance of the source of the prefetch request. In anembodiment, if the importance of the source exceeds a threshold and ifthe prefetch request is in address conflict with a transactional memoryoperation, prefetch manager 2208 queues the prefetch operation andotherwise the prefetch request is discarded.

In an embodiment, when prefetch queue 2213 is full, an arriving prefetchrequest is discarded. In an embodiment, when an incoming prefetchrequest is discarded because prefetch queue 2213 is full, a response tothe prefetch request is sent to the source of the prefetch request thatinforms it that the prefetch request was discarded because prefetchqueue 2213 was full. In an embodiment, if prefetch queue 2213 is full,an entry from prefetch queue 2213 is selected and discarded. In anembodiment, queue manager 2214 selects a prefetch in prefetch queue 2213that is to be discarded. In an embodiment, if queue manager 2214discards a prefetch from prefetch queue 2213 to make room for anotherprefetch request, the source of the discarded prefetch is notified thatthe prefetch request was discarded to make room for another prefetchrequest. In an embodiment, if a prefetch request causes an expulsionfrom prefetch queue 2213, the source of the prefetch request is notifiedthat the prefetch request caused an expulsion from prefetch queue 2213.In an embodiment, if an expulsion from the prefetch queue is performed,the oldest entry in the prefetch queue is discarded. In anotherembodiment, the prefetch request that is discarded from prefetch queue2213 is the prefetch request at the head of prefetch queue 2213. Inanother embodiment, the prefetch request in prefetch queue 2213 that isdiscarded is the prefetch request with a shortest time horizon, that isthe prefetch request with the soonest end time. In another embodiment,the prefetch request selected from the queue to be discarded is theprefetch request with the greatest time horizon, that is, the prefetchrequest with an end time that is farthest into the future.

In other embodiments, various buses in FIG. 5 are combined. Thus, in oneor more of such embodiments, as non-limiting examples, the function ofread address bus 2201 and prefetch read address bus 2203 are performedby a single read bus with a request type indicator; the function ofwrite address buss 2202 and prefetch write bus 2204 are performed by asingle write bus with a request type indicator; the function of prefetchbuses 2203 and 2204 are performed by a single prefetch bus with arequest type indicator; the functions of non-prefetch buses 2201 and2203 are performed by a single non-prefetch bus with a bus indicator;the functions of all buses are performed by a single address bus with arequest type indicator. In other embodiments, request and address busesare combined.

In some scenarios and embodiments, a prefetch protocol includes a readrequest that is asserted on a bus based or switch based interconnectthat includes an information as to a purpose of a prefetch request. Inan embodiment, in a system that incorporates a MESI cache coherencyprotocol, the cache coherency protocol is enhanced to include a readrequest that includes a specification as to whether the read request isa demand request (i.e., a non-prefetch request for data) or a prefetchrequest. In an embodiment, a prefetch request is a prefetch forexclusive ownership of a data (e.g., to read or write a data) or aprefetch for shared ownership of a data (e.g., to read the data).

FIG. 6 depicts an embodiment of a format of a read request thatspecifies a demand request or a prefetch request and further specifieswhether a prefetch request is for exclusive ownership of a data or aprefetch for shared ownership of a data. In an embodiment, read request3311 is comprised of at least six information fields: read informationfield 3301, tag information field 3302, exclusive or shared informationfield 3303, p (prefetch) information field 3304, address informationfield 3305, and CRC (cyclic redundancy check) information field 3306. Inan embodiment, read information field 3301 identifies read request 3311as a read request; tag information field 3302 identifies a particularinstance of read request 3311; exclusive or shared information field3303 specifies whether read request 3311 is for an exclusive or sharedaccess to a data; p information field 3304 specifies whether or not readrequest 3311 is a prefetch request; address information field 3305specifies the address of the data requested by read request 3311; CRCinformation field 3306 contains a value that is computed from thebit-patterns of the values in the other information fields in readrequest 3311 and is used to detect an error that occurred during atransfer of read request 3311 from a source of read request 3311 to adestination of read request 3311.

FIG. 6 also depicts an embodiment of a format of a response to readrequest 3311, read response 3312. In an embodiment, read response 3312is comprised of at least four information fields: read responseinformation field 3307, tag information field 3308, data informationfield 3309, and CRC information field 3310. In an embodiment, readresponse information field 3307 identifies read response 3312 as a readresponse; tag information field 3308 identifies read response 3312 as aresponse to a read request 3311 with tag information field 3302 equal totag information field 3308; data information field 3309 contains a datathat is the data requested by a read request 3311 with a tag informationfield 3302 that is equal to tag information field 3308. CRC informationfield 3310 contains a value that is computed from the bit-patterns ofthe values in the other information fields in read response 3312 and isused to detect an error that occurred during a transfer of read response3312 from a source of read response 3312 to a destination of readresponse 3312.

In some scenarios and embodiments, a prefetch protocol includes aprefetch request that is asserted on a bus based or switch basedinterconnect that includes an information as to a purpose of a prefetchrequest. In an embodiment, in a system that incorporates a MESI cachecoherency protocol, the cache coherency protocol is enhanced to includea prefetch request that includes a specification as to whether theprefetch request is a prefetch for exclusive ownership of a data or aprefetch for shared ownership of a data.

FIG. 7 depicts an embodiment of a format of a prefetch request 4416 andan embodiment of a format for each of three types of response toprefetch request 4416: prefetch response 4417 (indicating that prefetchrequest 4416 was executed), prefetch reject response 4418 (indicatingthat prefetch request 4416 was rejected), and prefetch deferred response4419 (indicating that prefetch request 4416 was deferred). In anembodiment, prefetch request 4416 specifies whether a prefetch request4416 is for either exclusive ownership of a data or a prefetch forshared ownership of a data. In an embodiment, prefetch request 4416 iscomprised of at least five information fields: prefetch informationfield 4401, tag information field 4402, exclusive or shared informationfield 4403, address information field 4404, and CRC (cyclic redundancycheck) information field 4405. In an embodiment, prefetch informationfield 4401 identifies read request 4416 as a prefetch request; taginformation field 4402 identifies a particular instance of prefetchrequest 4416; exclusive or shared information field 4403 specifies thatprefetch request 4416 is either a prefetch request for exclusive accessor a prefetch for shared access; address information field 4404specifies the address of the data requested by prefetch request 4416;CRC information field 4405 contains a value that is computed from thebit-patterns of the values in the other information fields in prefetchrequest 4416 and is used to detect an error that occurred during atransfer of prefetch request 4416 from a source of prefetch request 4416to a destination of prefetch request 4416.

FIG. 7 also depicts an embodiment of a format for prefetch response4417. In an embodiment, prefetch response 4417 is sent in response to aprefetch request 4416, from a destination that received the prefetchrequest 4416, when the action in prefetch request 4416 is executed. Inan embodiment, prefetch response 4417 is comprised of at least fourinformation fields: prefetch response information field 4406, taginformation field 4407, data information field 4408, and CRC informationfield 4409. In an embodiment, prefetch response information field 4406identifies prefetch response 4417 as a prefetch response; taginformation field 4407 identifies prefetch response 4417 as a responseto a prefetch request 4416 with tag information field 4407 equal to taginformation field 4402; data information field 4408 contains a data thatis the data requested by a prefetch request 4416 with a tag informationfield 4402 that matches tag information field 4407; CRC informationfield 4409 contains a value that is computed from the bit-patterns ofthe values in the other information fields in prefetch response 4417 andis used to detect an error that occurred during a transfer of prefetchresponse 4417 from a source of prefetch response 4417 to a destinationof prefetch response 4417.

FIG. 7 also depicts an embodiment of a format of a second type ofresponse to prefetch request 4416, prefetch reject response 4418. In anembodiment, prefetch reject response 4418 is sent in response to aprefetch request 4416, from a destination that received the prefetchrequest 4416, when the action requested by prefetch request 4416 isrejected (i.e., aborted). In an embodiment, prefetch reject response4418 is comprised of at least three information fields: prefetch rejectinformation field 4410, tag information field 4411, and CRC informationfield 4412. In an embodiment, prefetch reject information field 4410identifies prefetch response 4418 as a prefetch reject response; taginformation field 4411 identifies prefetch reject response 4418 as aresponse to a prefetch request 4416 with a tag information field 4402equal to tag information field 4411; CRC information field 4409 containsa value that is computed from the bit-patterns of the values in theother information fields in prefetch reject response 4418 and is used todetect an error that occurred during a transfer of prefetch rejectresponse 4418 from a source of prefetch reject response 4418 to adestination of prefetch reject response 4418.

FIG. 7 also depicts an embodiment of a format of a third type ofresponse to prefetch request 4416, prefetch deferred response 4419. Inan embodiment, prefetch deferred response 4419 is sent in response to aprefetch request 4416, from a destination that received the prefetchrequest 4416, when the action requested by prefetch request 4416 isdeferred or delayed (e.g., queued in the destination). In an embodiment,prefetch deferred response 4419 is comprised of at least threeinformation fields: prefetch deferred information field 4413, taginformation field 4414, and CRC information field 4415. In anembodiment, prefetch deferred information field 4413 identifies prefetchdeferred response 4419 as a prefetch reject response; tag informationfield 4414 identifies prefetch deferred response 4419 as a response to aprefetch request 4416 with an tag information field 4402 equal to taginformation field 4414; CRC information field 4415 contains a value thatis computed from the bit-patterns of the values in the other informationfields in prefetch deferred response 4419 and is used to detect an errorthat occurred during a transfer of prefetch deferred response 4419 froma source of prefetch deferred response 4419 to a destination of prefetchdeferred response 4419.

In an embodiment, a prefetch protocol includes a packed prefetch requestthat is comprised of a plurality of prefetch operations, with eachprefetch operation having an associated address that is specified in thepacked prefetch request. In an embodiment, all the prefetch operationsin the packed prefetch request are of a same type of packed prefetchrequest. In an embodiment, a packed prefetch request can include eithera plurality of prefetch requests for exclusive ownership of a data or aplurality of prefetch requests for shared ownership of a data, but notboth. In an embodiment, each prefetch operation in the packed prefetchrequest can be a type of prefetch operation (e.g., a prefetch forexclusive ownership or a prefetch for shared ownership) that isindependent of a type of another prefetch operation included in thepacked prefetch request.

In an embodiment, the one or more addresses in a packed prefetch requestare in a sequence of addresses and each address has a unique indexassociated with the address. In an embodiment, the value of an index ofan address (i.e., the address index) takes the value of the position ofthe address in the sequence of addresses in the packed prefetch request.In an embodiment, a packed prefetch request includes a packed prefetchrequest tag that identifies the packed prefetch request. In anembodiment, the packed prefetch request tag is comprised of a unique tagfor the packed prefetch request and a unique tag for each address in thesequence of addresses.

In an embodiment, a response to a packed prefetch request (e.g., aresponse with data, a response indicating a rejection of a prefetchoperation, or a response indicating that an operation has been deferred)in the prefetch protocol is associated with an prefetch operationincluded in the packed prefetch request. The response includes the tagof the packed prefetch request and an identifier of the address (i.e.,an address identifier) of the prefetch operation in the packed prefetchrequest to which the response is associated. Since a packed prefetchrequest can incorporate one or more prefetch operations, a response toan operation in the packed prefetch request must identify the individualprefetch operation to which the response is associated. In anembodiment, the address identifier is the index of an address in thesequence of one or more addresses in the packed prefetch request. In anembodiment, the address identifier is a unique tag for the address thatis included in the packed prefetch tag of the packed prefetch request.

FIG. 8 depicts an embodiment of a format of a packed prefetch request5501 and an embodiment of a format for each of three types of responseto packed prefetch request 5501: packed prefetch response 5502(indicating that a prefetch operation in packed prefetch request 5501was executed), packed prefetch reject 5503 (indicating that a prefetchoperation in packed prefetch request 5501 was rejected), and packedprefetch deferred 5504 (indicating that a prefetch operation in packedprefetch request 5501 was deferred). In an embodiment, packed prefetchrequest 5501 is comprised of at least five information fields: packedprefetch information field 5505, packed prefetch tag information field5506, exclusive or shared information field 5507, address informationfield 5508, and CRC (cyclic redundancy check) information field 5501. Inan embodiment, packed prefetch information field 5505 identifies packedprefetch request 5501 as a packed prefetch request; packed taginformation field 5506 identifies a particular instance of packedprefetch request 5501; exclusive or shared information field 5507specifies that prefetch request 5501 is either a prefetch request forexclusive access or a prefetch for shared access; address informationfield 5508 specifies one or more addresses of the data requested bypacked prefetch request 5501; CRC information field 5509 contains avalue that is computed from the bit-patterns of the values in the otherinformation fields in packed prefetch request 5509 and is used to detectan error that occurred during a transfer of packed prefetch request 5509from a source of prefetch request 5509 to a destination of packedprefetch request 5509. In an embodiment, packed tag information field5506 includes a unique tag for packed prefetch 5501 and a unique tag foreach address in address information field 5508.

FIG. 8 also depicts an embodiment of a format of packed prefetchresponse 5502, which is a response to a prefetch operation in packedprefetch 5501. In an embodiment, packed prefetch response 5502 is sentfrom a destination that received a packed prefetch request 5501 inresponse to the packed prefetch request 5501, when a prefetch operationin packed prefetch request 5501 is executed. In an embodiment, packedprefetch response 5502 is comprised of at least four information fields:packed prefetch response information field 5513, operation taginformation field 5510, data information field 5514, and CRC informationfield 5515. In an embodiment, packed prefetch response information field5513 identifies packed prefetch response 5502 as a packed prefetchresponse; operation tag information field 5510 identifies packedprefetch response 5502 as a response to a prefetch operation in packedprefetch request 5501 with packed prefetch tag 5506; data informationfield 5514 contains a data that is the data requested by a prefetchoperation in prefetch request 5501 with a operation tag informationfield 5510 that includes the unique tag that identifies a uniqueinstance of packed prefetch 5501 and an address identifier thatidentifies a unique prefetch operation within the unique instance ofpacked prefetch 5501; CRC information field 5515 contains a value thatis computed from the bit-patterns of the values in the other informationfields in packed prefetch response 5502 and is used to detect an errorthat occurred during a transfer of packed prefetch response 5502 from asource of packed prefetch response 5502 to a destination of packedprefetch response 5502. In an embodiment, the address identifier inoperation tag 5510 is a unique address tag that is included in packedprefetch tag information field 5506. In an embodiment, the addressidentifier in operation tag 5510 is an index of an address that isincluded in address information field 5508.

FIG. 8 also depicts an embodiment of a format of a second type ofresponse to packed prefetch request 5501, packed prefetch rejectresponse 5503. In an embodiment, packed prefetch reject response 5503 issent in response to a prefetch operation in a packed prefetch request5501, from a destination that received the packed prefetch request 5501,when a prefetch operation included in packed prefetch request 5501 isrejected (i.e., aborted). In an embodiment, packed prefetch rejectresponse 5503 is comprised of at least three information fields: packedprefetch reject response information field 5516, operation taginformation field 5511, and CRC information field 5517. In anembodiment, packed prefetch reject information field 5516 identifiesprefetch reject response 5503 as a prefetch reject response; operationtag information field 5511 identifies packed prefetch reject response5503 as a response to a prefetch operation included in packed prefetchrequest 5501 with a operation tag information field 5511 that includesthe unique tag that identifies a unique instance of packed prefetchrequest 5501 and an address identifier that identifies a unique prefetchoperation; CRC information field 5517 contains a value that is computedfrom the bit-patterns of the values in the other information fields inpacked prefetch reject response 5503 and is used to detect an error thatoccurred during a transfer of packed prefetch reject response 5503 froma source of packed prefetch reject response 5503 to a destination ofpacked prefetch reject response 5503. In an embodiment, the addressidentifier in operation tag 5511 is a unique address tag that isincluded in packed prefetch tag information field 5506. In anembodiment, the address identifier in operation tag 5511 is an index ofan address that is included in address information field 5508.

FIG. 8 also depicts an embodiment of a format of a third type ofresponse to packed prefetch request 5501, packed prefetch deferredresponse 5504. In an embodiment, packed prefetch deferred response 5504is sent in response to a prefetch operation in a packed prefetch request5501, from a destination that received the packed prefetch request 5501,when a prefetch operation included in packed prefetch request 5501 isdeferred (e.g., queued for a delayed execution or delayed untilspecified conditions in a local processor or in a remote processor aremet). In an embodiment, packed prefetch deferred response 5504 iscomprised of at least three information fields: packed prefetch deferredresponse information field 5518, operation tag information field 5512,and CRC information field 5519. In an embodiment, packed prefetchdeferred information field 5518 identifies prefetch deferred response5504 as a prefetch deferred response; operation tag information field5512 identifies packed prefetch deferred response 5504 as a response toa prefetch operation included in packed prefetch request 5501 with aoperation tag information field 5512 that includes the unique tag thatidentifies a unique instance of packed prefetch 5501 and an addressidentifier that identifies a unique prefetch operation; CRC informationfield 5519 contains a value that is computed from the bit-patterns ofthe values in the other information fields in packed prefetch deferredresponse 5504 and is used to detect an error that occurred during atransfer of prefetch reject response 5503 from a source of packedprefetch deferred response 5504 to a destination of packed prefetchdeferred response 5504. In an embodiment, the address identifier inoperation tag 5511 is a unique address tag that is included in packedprefetch tag information field 5506. In an embodiment, the addressidentifier in operation tag 5511 is an index of an address that isincluded in address information field 5508.

In other embodiments, a prefetch protocol includes additional responsesto prefetch request 4416 and to packed prefetch request 5501. In anembodiment, a cache that is associated with a source of a prefetchoperation marks data that the cache receives from a prefetch operationas prefetched data. In an embodiment, the cache uses the information onprefetched data that it contains in the management of the memory spaceof the cache (e.g., to determine which data to evict to accommodate newdata).

FIG. 9 is a flowchart that depicts the operation of transactional memorysystem A 1104 in an embodiment. In an embodiment, transactional memorysystem A 1104 receives a protocol request for memory operation onnon-prefetch operations bus 2210 or on prefetch operations bus 2209(step 301). Abort manager 2207 determines whether the memory operationis a read, i.e., a read type of memory operation (decision step 302). Ifthe memory operation is a read (decision step 302, YES branch), thenabort manager 2207 determines whether the address of the read matches anaddress in write set 2205 of a transaction (decision step 303). If theaddress of the read does not match an address in write set 2205 of atransaction (decision step 303, NO branch), then the read operation isperformed and, for remote requests, a response is sent by abort manager2207 in accordance with an exemplary protocol of FIG. 7 (step 304) andthe process terminates (step 313).

If the address of the read matches an address in write set 2205 of atransaction (decision step 303, YES branch), then abort manager 2207determines whether the read is a prefetch operation (decision step 305).If the read is a prefetch operation (decision step 305, YES branch),then the read prefetch is managed by prefetch manager 2208 (step 312),and the process terminates (step 313). If the read is not a prefetchoperation (decision step 305, NO branch), then the transaction that isassociated with the address of the read is aborted (step 307), and theprocess terminates (step 313).

If the memory operation is not a read (decision step 302, NO branch),then abort manager 2207 determines that the memory operation is a writeoperation and abort manager 2207 determines whether the address of thewrite operation matches an address in read set 2206 of a transaction(decision step 308). If the address of the write operation does notmatch an address in read set 2206 of a transaction (decision step 308,NO branch), then abort manager 2207 determines whether the address ofthe write operation matches an address in write set 2205 of atransaction (decision step 309). If the address of the write operationdoes not match an address in write set 2205 of a transaction (decisionstep 309, NO branch), then the write operation is executed, (i.e., thewrite is performed for a local write request, and if the write requestoriginated in a remote processor or node, then the appropriate cacheline is transferred with exclusive ownership to the remote processor)(step 314) and the process terminates (step 313). If the address of thewrite operation matches an address in write set 2205 of a transactionalmemory operation (decision step 309, YES branch), then abort manager2207 determines whether the write operation is a prefetch operation(decision step 310). If the address of the write operation matches anaddress in read set 2206 of a transaction (decision step 308, YESbranch), then abort manager 2207 determines whether the write operationis a prefetch operation (decision step 310). If abort manager 2207determines that the write operation is not a prefetch operation (step310, NO branch), then the transaction that is associated with theaddress of the write operation is aborted by abort manager 2207 (step311), and the process terminates (step 313). If the write operation isdetermined to be a prefetch for write operation (decision step 310, YESbranch), then the write prefetch is managed by prefetch manager 2208(step 312), and the process terminates (step 313).

FIG. 10 is a flowchart that depicts the major operations of prefetchmanager 2208 to manage a prefetch operation that occur within the manageprefetch step 312 of FIG. 9. Local-remote 2216 in prefetch manager 2208determines whether the prefetch operation is local or remote (decisionstep 401). If the prefetch operation is local (decision step 401, YESbranch), then the prefetch operation is performed (step 402) and theprocess terminates (step 419). (In at least one embodiment, performing aprefetch includes indicating a successful prefetch with no furtheraction, as when the requester and requestee are accessing the samecache.) If local-remote 2216 determines that the prefetch operation isremote (decision step 401, NO branch), then queue manager 2214determines whether the prefetch operation should be inserted intoprefetch queue 2213 (decision step 420). If queue manager 2214determines not to insert the prefetch operation into prefetch queue 2213(step 420, NO branch), then prefetch manager 2208 discards the prefetchoperation (step 413). Prefetch manager 2208 determines whether thesource of the prefetch operation should be notified that the prefetchoperation was discarded (decision step 414). If prefetch manager 2208determines that the source of the prefetch operation should be notifiedthat the prefetch operation was discarded (decision step 414, YESbranch), then the source of the prefetch operation is notified byprefetch manager 2208 (step 415) and operations in prefetch manager 2208complete (step 419). If prefetch manager 2208 determines that the sourceof the prefetch operation should not be notified that the prefetchoperation was discarded (decision step 414, NO branch), then operationsin prefetch manager 2208 complete (step 419).

If queue manager 2214 determines that the prefetch operation is to beinserted into prefetch queue 2213 (decision step 420, YES branch), thenqueue manager 2214 determines whether prefetch queue 2213 is full(decision step 421). If prefetch queue 2213 is not full (decision step421, NO branch), then queue manager 2214 inserts the prefetch operationinto prefetch queue 2213 (step 416). Prefetch manager 2208 determineswhether to inform the source of the prefetch operation that the prefetchoperation was queued (decision step 417). If prefetch manager 2208determines that the source of the prefetch operation should be notified(decision step 417, YES branch), then prefetch manager 2208 notifies thesource of the prefetch operation (step 418), after which the processcompletes (step 419). If prefetch manager 2208 determines that thesource of the prefetch operation should not be notified (decision step417, NO branch), then the process completes (step 419).

If queue manager 2214 determines that prefetch queue 2213 is full(decision step 421, YES branch), then queue manager 2214 determineswhether to evict an entry from prefetch queue 2213 (decision step 403).If queue manager 2214 determines that an entry is to be evicted(decision step 403, YES branch), then queue manager 2214 evicts an entryfrom prefetch queue 2215 (step 407). Prefetch manager 2208 determineswhether to notify the source of the evicted prefetch operation of theeviction of the prefetch operation (decision step 408). If prefetchmanager 2208 determines to notify the source of the evicted prefetchoperation (decision step 408, YES branch), then the source of theevicted prefetch operation is notified of the eviction by prefetchmanager 2208 (step 409) and the new prefetch operation is queued inprefetch queue 2213 by prefetch manager 2208 (step 410). If prefetchmanager 2208 determines that the source of the evicted prefetchoperation is not to be notified (decision step 408, NO branch), then thenew prefetch operation is queued in prefetch queue 2213 by prefetchmanager 2208 (step 410). Prefetch manager 2208 determines whether tonotify the source of the new prefetch operation of the queuing of thenew prefetch operation (decision step 411). If prefetch manager 2208determines that the source of the new prefetch operation is to benotified of the queuing of the new prefetch operation (decision step411, YES branch), then the source of the new prefetch operation isnotified, by prefetch manager 2208, thereby indicating that the newprefetch operation was queued (step 412) and the process completes (step419). If prefetch manager 2208 determines that the source of the newprefetch operation is not to be notified (decision step 411, NO branch),then the process completes (step 419).

If queue manager 2214 determines that a prefetch operation is not to beevicted (decision step 403, NO branch), then the new prefetch operationis discarded by prefetch manager 2208 (step 404). Prefetch manager 2208determines whether to notify the source of the discarded prefetchoperation of the discarding of the prefetch operation (decision step405). If prefetch manager 2208 determines that the source of thediscarded prefetch operation should be notified that the prefetchoperation was discarded (decision step 405, YES branch), then the sourceof the discarded prefetch operation is notified of that discarding byprefetch manager 2208 (step 406) and the process completes (step 419).If prefetch manager 2208 determines that the source of the evictedprefetch operation is not to be notified of the discarding of theprefetch operation (decision step 405, NO branch), then the processcompletes (step 419).

If queue manager 2214 determines that prefetch queue 2213 is not full(decision step 402, NO branch), then the prefetch operation is queued byqueue manager 2214 (step 416) and prefetch manager 2208 determineswhether the source of the prefetch operation should be notified that theprefetch operation was queued (decision step 417). If prefetch manager2208 determines that the source of the prefetch operation is to benotified that the prefetch operation was queued (decision step 417, YESbranch), then prefetch manager 2208 notifies the source of the prefetchoperation thereby indicating that the prefetch operation was queued(step 418) and the process completes (step 419). If prefetch manager2208 determines that the source of the prefetch operation is not to benotified that the prefetch operation was queued (decision step 417, NObranch), the process completes (step 419).

In accordance with one aspect of one embodiment, queued prefetches areperformed when the conflicting transaction terminates, either by way ofsuccessful completion, or by way of a transaction abort.

In at least one embodiment of prefetch manager 2208, the determinationsof whether to perform notifications responsive to discarding, queuing,evicting, and so forth, of prefetch requests is set at a design time ofprefetch manager 2208 and incorporated into the logic of prefetchmanager 2208. In one embodiment, such determinations correspond to oneor more control values, which may be configured using a variety of ways,such as software-writeable control registers, firmware-writeable controlregisters, control values configured at boot time, e.g., from aread-only storage (such as a boot EPROM), or by other reversible orirreversible means (e.g., by severing an electric connection at chipproduction time using a fuse or other means for controllablyinterrupting an electric connection, the presence or absence of saidconnection being indicative of a selection).

FIG. 8 depicts computer system 800 that is an example of a system thatincludes computer system 1101. Processors 804 and cache 816 aresubstantially equivalent to processor A 1102, cache system A 1103,processor B 1106, and cache system B 1107. Computer system 800 includescommunications fabric 802, which provides communications betweencomputer processor(s) 804, memory 806, persistent storage 808,communications unit 810, and input/output (I/O) interface(s) 812.Communications fabric 802 can be implemented with any architecturedesigned for passing data and/or control information between processors(such as microprocessors, communications and network processors, etc.),system memory, peripheral devices, and any other hardware componentswithin a system. For example, communications fabric 402 can beimplemented with one or more buses.

Memory 806 and persistent storage 808 are computer readable storagemedia. In this embodiment, memory 806 includes random access memory(RAM). In general, memory 806 can include any suitable volatile ornon-volatile computer readable storage media. Cache 816 is a fast memorythat enhances the performance of processors 804 by holding recentlyaccessed data and data near accessed data from memory 806.

Program instructions and data used to practice embodiments of thepresent invention may be stored in persistent storage 808 for executionby one or more of the respective processors 804 via cache 816 and one ormore memories of memory 806. In an embodiment, persistent storage 808includes a magnetic hard disk drive.

Alternatively, or in addition to a magnetic hard disk drive, persistentstorage 808 can include a solid state hard drive, a semiconductorstorage device, read-only memory (ROM), erasable programmable read-onlymemory (EPROM), flash memory, or any other computer readable storagemedia that is capable of storing program instructions or digitalinformation.

The media used by persistent storage 808 may also be removable. Forexample, a removable hard drive may be used for persistent storage 808.Other examples include optical and magnetic disks, thumb drives, andsmart cards that are inserted into a drive for transfer onto anothercomputer readable storage medium that is also part of persistent storage808.

Communications unit 810, in these examples, provides for communicationswith other data processing systems or devices. In these examples,communications unit 810 includes one or more network interface cards.Communications unit 810 may provide communications through the use ofeither or both physical and wireless communications links. Programinstructions and data used to practice embodiments of the presentinvention may be downloaded to persistent storage 808 throughcommunications unit 810.

I/O interface(s) 812 allows for input and output of data with otherdevices that may be connected to each computer system. For example, I/Ointerface 812 may provide a connection to external devices 818 such as akeyboard, keypad, a touch screen, and/or some other suitable inputdevice. External devices 818 can also include portable computer readablestorage media such as, for example, thumb drives, portable optical ormagnetic disks, and memory cards. Software and data used to practiceembodiments of the present invention can be stored on such portablecomputer readable storage media and can be loaded onto persistentstorage 808 via I/O interface(s) 812. I/O interface(s) 812 also connectto a display 820.

Display 820 provides a mechanism to display data to a user and may be,for example, a computer monitor.

The programs described herein are identified based upon the applicationfor which they are implemented in a specific embodiment of theinvention. However, it should be appreciated that any particular programnomenclature herein is used merely for convenience, and thus theinvention should not be limited to use solely in any specificapplication identified and/or implied by such nomenclature.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The terminology used herein was chosen to best explain the principles ofthe embodiment, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise.

Each respective figure, in addition to illustrating methods of andfunctionality of the present invention at various stages, alsoillustrates the logic of the method as implemented, in whole or in part,by one or more devices and structures. Such devices and structures areconfigured to (i.e., include one or more components, such as resistors,capacitors, transistors and the like that are connected to enable theperforming of a process) implement the method of merging one or morenon-transactional stores and one or more thread-specific transactionalstores into one or more cache line templates in a store buffer in astore cache. In other words, one or more computer hardware devices canbe created that are configured to implement the method and processesdescribed herein with reference to the Figures and their correspondingdescriptions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiment, the practical application or technicalimprovement over technologies found in the marketplace, or to enableother of ordinary skill in the art to understand the embodimentsdisclosed herein.

Embodiments of the present invention may be used in a variety ofelectronic applications, including but not limited to advanced sensors,memory/data storage, semiconductors, microprocessors and otherapplications.

A resulting device and structure, such as an integrated circuit (IC)chip can be distributed by the fabricator in raw wafer form (that is, asa single wafer that has multiple unpackaged chips), as a bare die, or ina packaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention maybe included by only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description. Areference to an element in the singular is not intended to mean “one andonly one” unless specifically stated, but rather “one or more.” Allstructural and functional equivalents to the elements of the variousembodiments described throughout this disclosure that are known or latercome to be known to those of ordinary skill in the art are expresslyincorporated herein by reference and intended to be encompassed by theinvention. It is therefore to be understood that changes may be made inthe particular embodiments disclosed which are within the scope of thepresent invention as outlined by the appended claims.

What is claimed is:
 1. A method to modify prefetch request processing,the method comprising: receiving, by a local processor of a group of oneor more processors of a local computer, a prefetch request from a remoteprocessor of a remote computer that is in communication with the localcomputer; prior to execution of program instructions included in theprefetch request, determining, by the group of one or more processors,whether execution of the prefetch request is predicted to cause anaddress conflict by accessing a memory address that is accessed duringan execution of a transaction of the local processor; responsive to adetermination that execution of the prefetch request is predicted tocause a conflict during the execution of the transaction, comparing, bythe group of one or more processors, a priority of the prefetch requestwith a priority of the transaction; and based, at least in part, on aresult of the comparison, modifying, by the group of one or moreprocessors, one or more program instructions that govern execution ofthe program instructions included in the prefetch request to includeprogram instruction to perform one or both of: (i) a quiesce of theprefetch request prior to execution of the prefetch request, (ii) adelay in execution of the prefetch request for a predetermined delayperiod.
 2. The method of claim 1, further comprising: responsive to atleast one of (i) a determination that the local processor has notransaction, and (ii) a determination that an execution of the prefetchrequest is not predicted to cause the conflict during the execution ofthe transaction of the local processor, executing, by the group of oneor more processors, the prefetch request, wherein the prefetch requestis one of a read prefetch instruction or a write prefetch instruction.3. The method of claim 1, wherein quiescing the prefetch requestcomprises: monitoring, by the group of one or more processors, one ormore conditions in the local processor and executing the prefetchrequest when the one or more conditions meet a criterion.
 4. The methodof claim 1, further comprising: responsive to the determination thatexecution of the prefetch request is predicted to cause the conflictduring the execution of the transaction, delaying, by the group of oneor more processors, execution of the prefetch request for the delayperiod, wherein the delay period is based on a prefetch time horizonthat is associated with a prefetch operation that is divided into timesteps with each time step specified in a preset number of cycles that areceiver of the prefetch request uses to manage the prefetch request;and performing, by the group of one or more processors, an action at anend of the delay period that is based on one or more conditions in thelocal processor, wherein the action includes one or more of (i) abortingthe prefetch request, (ii) further delaying execution of the prefetchrequest, (iii) quiescing the prefetch request, and (iv) executing theprefetch request.
 5. The method of claim 4, further comprising:notifying, by the group of one or more processors, the remote processorof the action taken at the end of the delay period.
 6. The method ofclaim 4, wherein delaying the prefetch request comprises: queuing, bythe group of one or more processors, the prefetch request in a queue ofa group of one or more queues.
 7. The method of claim 6, furthercomprising: notifying, by the group of one or more processors, theremote processor that the prefetch request was queued.
 8. The method ofclaim 5, wherein each queue in the one or more queues holds all prefetchrequests from a same remote processor and only prefetch requests fromthe same remote processor.
 9. The method of claim 5, wherein the groupof one or more queues includes one or both of: (i) at least one queue,of the one or more queues, that holds all prefetch requests forexclusive ownership and only prefetch requests for exclusive ownership,and (ii) at least one queue, of the one or more queues, that holds allprefetch requests for shared ownership and only prefetch requests forshared ownership.
 10. A computer program product to modify prefetchrequest processing, the program instructions comprising: one or morecomputer readable storage media and program instructions stored on theone or more computer readable storage media, the program instructionscomprising: program instructions to receiving, by a local processor of agroup of one or more processors of a local computer, a prefetch requestfrom a remote processor of a remote computer that is in communicationwith the local computer; program instructions to, prior to execution ofprogram instructions included in the prefetch request, determine whetherexecution of the prefetch request is predicted to cause an addressconflict by accessing a memory address that is accessed during anexecution of a transaction of the local processor; program instructionsto respond to a determination that execution of the prefetch request ispredicted to cause a conflict during the execution of the transaction,comparing a priority of the prefetch request with a priority of thetransaction; and program instructions to, based, at least in part, on aresult of the comparison, modify one or more program instructions thatgovern execution of the program instructions included in the prefetchrequest to include program instruction to perform one or both of: (i) aquiesce of the prefetch request prior to execution of the prefetchrequest, and (ii) a delay in execution of the prefetch request for apredetermined delay period.
 11. The computer program product of claim10, further comprising: program instructions to respond to at least oneof (i) a determination that the local processor has no transaction, and(ii) a determination that an execution of the prefetch request is notpredicted to cause the conflict during the execution of the transactionof the local processor, by executing the prefetch request, wherein theprefetch request is one of a read prefetch instruction or a writeprefetch instruction.
 12. The computer program product of claim 10,wherein quiescing the prefetch request includes monitoring one or moreconditions in the local processor and executing the prefetch requestwhen the one or more conditions meet a criterion.
 13. The computerprogram product of claim 10, further comprising: program instructions torespond to the determination that execution of the prefetch request ispredicted to cause the conflict during the execution of the transactionby delaying execution of the prefetch request for the delay period,wherein the delay period is based on a prefetch time horizon that isassociated with a prefetch operation that is divided into time stepswith each time step specified in a preset number of cycles that areceiver of the prefetch request uses to manage the prefetch request;and program instructions to perform an action at an end of the delayperiod that is based on one or more conditions in the local processor,wherein the action includes one or more of (i) aborting the prefetchrequest, (ii) further delaying execution of the prefetch request, (iii)quiescing the prefetch request, and (iv) executing the prefetch request.14. The computer program product of claim 13, further comprising:program instructions to notify the remote processor of the action takenat the end of the delay period.
 15. The computer program product ofclaim 13, wherein delaying the prefetch request comprises queuing theprefetch request in a queue in a group of one or more queues.
 16. Thecomputer program product of claim 15, further comprising: programinstructions to notify the remote processor that the prefetch requestwas queued.
 17. The computer program product of claim 14, wherein thegroup of one or more queues includes one or both of: (i) at least onequeue, of the one or more queues, that holds all prefetch requests forexclusive ownership and only prefetch requests for exclusive ownership,and (ii) at least one queue, of the one or more queues, that holds allprefetch requests for shared ownership and only prefetch requests forshared ownership.
 18. The computer program product of claim 10, whereinquiescing the prefetch request includes monitoring one or moreconditions in the local processor and executing the prefetch requestwhen the one or more conditions meet a criterion.
 19. A computer systemto modify prefetch request processing, the computer system comprising:one or more computer processors; one or more computer readable storagemedium; program instructions stored on the computer readable storagemedium for execution by at least one of the one or more processors, theprogram instructions comprising: program instructions to receiving, by alocal processor of a group of one or more processors of a localcomputer, a prefetch request from a remote processor of a remotecomputer that is in communication with the local computer; programinstructions to, prior to execution of program instructions included inthe prefetch request, determine whether execution of the prefetchrequest is predicted to cause an address conflict by accessing a memoryaddress that is accessed during an execution of a transaction of thelocal processor; program instructions to respond to a determination thatexecution of the prefetch request is predicted to cause a conflictduring the execution of the transaction, comparing a priority of theprefetch request with a priority of the transaction; and programinstructions to, based, at least in part, on a result of the comparison,modify one or more program instructions that govern execution of theprogram instructions included in the prefetch request to include programinstruction to perform one or both of: (i) a quiesce of the prefetchrequest prior to execution of the prefetch request, and (ii) a delay inexecution of the prefetch request for a predetermined delay period. 20.The computer system of claim 19, further comprising: programinstructions to respond to at least one of (i) a determination that thelocal processor has no transaction, and (ii) a determination that anexecution of the prefetch request is not predicted to cause the conflictduring the execution of the transaction of the local processor, byexecuting the prefetch request, wherein the prefetch request is one of aread prefetch instruction or a write prefetch instruction.